Modern websites, mobile apps, and business software rarely operate completely on their own. Most applications need to communicate with other systems, retrieve information, process payments, send messages, or synchronize data.
One of the most common ways software systems communicate is through a REST API.
REST APIs are used by everything from mobile apps and e-commerce websites to accounting platforms, CRMs, payment processors, and artificial intelligence services. If you have ever logged into an application using Google, checked a shipping status, paid for something online, or used a mobile app that retrieves information from a server, there is a good chance a REST API was involved.
But what exactly is a REST API, and how does it work?
What Is an API?
Before explaining REST APIs, it helps to understand what an API is.
API stands for Application Programming Interface.
An API provides a structured way for one piece of software to communicate with another.
Instead of allowing an application to directly access another application’s database or internal code, the API provides specific endpoints that outside applications can use.
For example, a weather service might provide an API that allows another application to request the current temperature for Austin, Texas.
The requesting application does not need to know how the weather company stores its data or how its internal software works. It simply sends a request to the API and receives the information it needs.
APIs create a controlled bridge between different software systems.
What Does REST Mean?
REST stands for Representational State Transfer.
REST is an architectural approach for designing web-based APIs. It was introduced by computer scientist Roy Fielding in his doctoral dissertation in 2000.
A REST API typically uses standard web technologies and HTTP requests to allow applications to communicate with one another.
Because REST APIs use many of the same technologies that power ordinary websites, they are relatively straightforward for developers to build, test, and integrate.
REST has become one of the most widely used approaches for building APIs.
How Does a REST API Work?
A REST API generally follows a request-and-response model.
An application sends a request to an API endpoint.
The server receives the request, performs the necessary operation, and sends back a response.
For example, imagine a mobile application that displays customer information.
The app might send a request such as:
GET /customers/125
The API receives the request and looks for customer number 125.
It might then return data similar to:
{
"id": 125,
"name": "John Smith",
"email": "john@example.com"
}
The mobile application can then display that information to the user.
This exchange can happen in a fraction of a second.
What Is a REST API Endpoint?
An endpoint is a specific location within an API where an application can request or modify information.
Endpoints are usually represented by URLs.
For example:
https://api.example.com/customers
might return a list of customers.
While:
https://api.example.com/customers/125
might return information about one specific customer.
A larger application could have dozens or even hundreds of API endpoints.
Examples might include:
/customers
/orders
/products
/payments
/invoices
/users
Each endpoint typically performs a specific function.
Common REST API Methods
REST APIs generally use standard HTTP methods to tell the server what type of action should be performed.
GET
GET retrieves information.
For example:
GET /products
could return a list of products.
POST
POST usually creates a new record.
For example:
POST /customers
could create a new customer.
PUT
PUT is commonly used to update or replace an existing record.
For example:
PUT /customers/125
could update customer number 125.
PATCH
PATCH is also used for updates but generally changes only specific parts of an existing record.
For example, a PATCH request might update a customer’s phone number without changing their name or email address.
DELETE
DELETE removes a record or resource.
For example:
DELETE /customers/125
could delete customer number 125.
Together, these methods give developers a standardized way to create, retrieve, update, and delete information.
REST APIs and CRUD Operations
Developers frequently use the term CRUD when discussing databases and APIs.
CRUD stands for:
Create
Read
Update
Delete
These operations closely correspond with common REST API methods.
POST is commonly used to create records.
GET retrieves or reads records.
PUT or PATCH updates records.
DELETE removes records.
This predictable structure is one reason REST APIs are relatively easy for developers to understand.
What Format Do REST APIs Use?
Most modern REST APIs exchange information using JSON.
JSON stands for JavaScript Object Notation.
A typical JSON response might look like this:
{
"product": "Laptop",
"price": 1299,
"in_stock": true
}
JSON is popular because it is lightweight, readable, and supported by nearly every modern programming language.
Older APIs may use XML, and some REST APIs support multiple data formats, but JSON has become the most common format.
What Are REST APIs Used For?
REST APIs can connect almost any type of software.
Mobile Applications
Many mobile apps use REST APIs to communicate with cloud-based servers.
The mobile application handles the user interface while the REST API manages information stored on the server.
For example, a restaurant ordering app might use an API to retrieve menu items, submit orders, process payments, and display order status.
Website Integrations
Websites frequently use REST APIs to communicate with third-party services.
Examples include connecting a website to:
- Customer relationship management systems
- Accounting software
- Shipping platforms
- Marketing platforms
- Payment gateways
- Inventory systems
- Mapping services
Payment Processing
Payment providers frequently offer APIs that developers can integrate into websites and applications.
An application can send payment information to the payment provider’s API and receive a response indicating whether the payment was successful.
The business application does not need to build an entire payment processing network itself.
CRM Integrations
REST APIs are commonly used to connect websites and custom applications to CRM platforms.
A website contact form, for example, could automatically send a new lead into a company’s CRM.
The CRM could then trigger additional actions such as assigning a salesperson, sending an email, or creating a follow-up task.
Artificial Intelligence
Many AI services are accessed through APIs.
A custom application might send information to an AI API and receive generated text, classifications, summaries, recommendations, or other results.
This allows businesses to add AI capabilities to existing applications without building the underlying AI models themselves.
E-Commerce
REST APIs play a major role in modern e-commerce systems.
An online store might use APIs for:
- Product inventory
- Payment processing
- Shipping calculations
- Sales tax calculations
- Customer accounts
- Order fulfillment
- Email notifications
Multiple systems can work together behind the scenes while appearing to customers as one application.
REST APIs vs. Direct Database Access
One of the major advantages of an API is that applications do not need direct access to another system’s database.
Direct database access can create security and compatibility problems.
Instead, an API defines exactly what information outside applications are allowed to request or change.
For example, an accounting system may allow an application to create an invoice through its API without providing direct access to every table in its accounting database.
This creates a much cleaner and safer integration.
How Are REST APIs Secured?
APIs frequently handle sensitive information, so security is extremely important.
Several methods can be used to protect REST APIs.
API Keys
An API key identifies the application making the request.
The API server checks the key before processing the request.
Access Tokens
Many modern APIs use access tokens.
A user or application authenticates first and receives a temporary token that is included with future API requests.
OAuth
OAuth is commonly used when one application needs permission to access information from another service on behalf of a user.
For example, clicking “Sign in with Google” often involves an OAuth authorization process.
HTTPS
REST APIs should normally use HTTPS so information transmitted between systems is encrypted.
Permissions
APIs can also restrict what different users or applications are allowed to do.
One application might have permission to retrieve customer information but not delete customers.
What Are REST API Status Codes?
REST APIs generally use standard HTTP status codes to indicate whether a request succeeded.
Some common examples include:
200 OK
The request was successful.
201 Created
A new resource was successfully created.
400 Bad Request
The request contained invalid information.
401 Unauthorized
Authentication is required or failed.
403 Forbidden
The user or application does not have permission to perform the requested action.
404 Not Found
The requested resource could not be found.
500 Internal Server Error
Something went wrong on the API server.
These codes help applications determine what happened and how they should respond.
What Is REST API Documentation?
Good API documentation explains how developers can communicate with an API.
Documentation typically includes:
- Available endpoints
- Required parameters
- Authentication instructions
- Example requests
- Example responses
- Error codes
- Rate limits
- Data formats
Developers often spend a significant amount of time reviewing API documentation when building integrations.
Well-designed documentation can dramatically reduce development time.
What Are API Rate Limits?
Many REST APIs limit how many requests an application can make during a specific period of time.
These restrictions are known as rate limits.
For example, an API might allow:
1,000 requests per hour.
Rate limits help protect API servers from excessive traffic and prevent individual applications from consuming too many resources.
Applications that regularly communicate with third-party APIs need to be designed with these limitations in mind.
REST API Integration Challenges
REST APIs make software integration easier, but integrations are not always simple.
Developers may encounter issues such as:
- Poor API documentation
- Authentication problems
- Inconsistent data formats
- Rate limits
- API version changes
- Missing functionality
- Slow API responses
- Service outages
- Duplicate records
- Unexpected error responses
A reliable integration should be designed to handle these situations gracefully.
For example, if a third-party API temporarily becomes unavailable, an application may need to retry the request rather than immediately failing.
What Happens When an API Changes?
Third-party APIs evolve over time.
A provider may introduce new features, change authentication requirements, or discontinue older API versions.
Well-managed APIs typically use version numbers such as:
/api/v1/customers
or:
/api/v2/customers
Versioning allows the API provider to introduce changes without immediately breaking applications using the older version.
However, businesses that depend heavily on third-party APIs should still monitor API changes and periodically update their integrations.
REST API vs. SOAP API
Before REST became dominant, SOAP was commonly used for web service integrations.
SOAP APIs generally rely on XML and follow stricter communication standards.
REST APIs tend to be simpler and more flexible.
SOAP is still used in some enterprise, banking, government, and legacy software environments, but REST is often preferred for modern web and mobile application development.
REST API vs. GraphQL
GraphQL is another approach to building APIs.
With a traditional REST API, developers usually access different endpoints for different types of information.
GraphQL often provides a single endpoint where developers can specify exactly which information they want returned.
GraphQL can be useful for complex applications, but REST remains extremely popular because of its simplicity, widespread support, and mature development tools.
Neither approach is automatically better. The right choice depends on the application.
Why Are REST APIs Important for Custom Software?
REST APIs allow custom software to communicate with existing business systems instead of recreating functionality that already exists.
A custom application might connect with several services at once.
For example, a business application could use:
- Stripe for payments
- QuickBooks for accounting
- HubSpot for CRM
- Google Maps for location services
- SendGrid for email
- Twilio for text messaging
- An AI platform for automated analysis
The custom application becomes the central interface while REST APIs handle communication between the different platforms.
This can greatly expand what an application can do without requiring every feature to be built from scratch.
Can You Build Your Own REST API?
Yes.
Businesses frequently develop private REST APIs for their own applications.
For example, a company might have a central database containing customers, orders, inventory, and pricing.
A REST API can sit between that database and multiple applications.
The company’s website, mobile app, employee dashboard, and partner systems could all communicate with the same API.
This creates a centralized software architecture where multiple systems can safely share information.
REST APIs can be created using many development technologies, including PHP, Laravel, Node.js, Python, Java, .NET, Ruby, and numerous other frameworks.
Do You Need an API Integration?
If two software systems need to exchange information automatically, an API integration may be the best solution.
Instead of employees repeatedly copying information between systems, an integration can perform the work automatically.
A properly designed REST API integration can:
- Reduce manual data entry
- Eliminate duplicate work
- Reduce human errors
- Synchronize information between systems
- Automate business processes
- Connect legacy systems with newer applications
- Add new features to existing software
For many businesses, API integrations are an important part of modernizing existing software without replacing every system they already use.
Final Thoughts
REST APIs are one of the technologies that quietly power much of the modern internet.
They allow websites, mobile applications, cloud services, payment platforms, databases, and business systems to communicate through a standardized interface.
While users may never see the APIs operating behind the scenes, developers rely on them every day to connect software and automate business processes.
Whether you are developing a new mobile application, integrating a CRM, connecting an accounting platform, adding artificial intelligence, or modernizing an older software system, REST APIs can provide the communication layer that brings everything together.
If your business needs a custom REST API, API integration, or assistance connecting existing software platforms, an experienced software development team can evaluate the available APIs and design an integration that fits your existing systems and business processes.
Frequently Asked Questions About REST APIs
What does REST API stand for?
REST stands for Representational State Transfer, while API stands for Application Programming Interface. A REST API is an API designed around REST architectural principles and commonly uses HTTP to exchange information between applications.
Are REST APIs only used for websites?
No. REST APIs are used by websites, mobile applications, desktop software, cloud services, internal business applications, Internet of Things devices, and many other types of software.
Do REST APIs always use JSON?
No, but JSON is the most common data format used by modern REST APIs. REST APIs can also exchange XML, text, or other data formats.
Are REST APIs secure?
REST APIs can be very secure when properly implemented. Common security measures include HTTPS encryption, authentication tokens, API keys, OAuth, access permissions, request validation, and rate limiting.
What programming languages can use REST APIs?
Almost every modern programming language can communicate with REST APIs, including PHP, JavaScript, Swift, Kotlin, Java, Python, C#, Ruby, and C++.
What is a REST API integration?
A REST API integration connects two or more software systems so they can automatically exchange information or trigger actions without requiring someone to manually transfer the data.
Can older software connect to a REST API?
Often, yes. REST APIs are frequently used to connect legacy applications with newer cloud platforms and services. Depending on the age and architecture of the older system, custom middleware or additional development may be required.