What Is an API? How Does It Work and What Is It Used For?

When a weather app displays the temperature in your city, an online store communicates with a payment provider or a mobile app retrieves user information from a server, an API is often working behind the scenes.

APIs are the invisible connection points of modern software. They allow one application to request data, start an operation or use a feature provided by another system in a controlled and predictable way.

This guide explains what an API is, how it works and where APIs are used without getting lost in unnecessary technical complexity.

What is an API?

API stands for Application Programming Interface.

In simple terms, an API is a set of rules that defines how two pieces of software communicate. When an application needs data or wants another system to perform an operation, it sends a request according to the rules defined by the API.

An API does not normally expose the other system’s entire source code or database. It provides controlled access only to the data and operations that its developers have chosen to make available.

How does an API work?

A common way to understand an API is to compare it with the relationship between a restaurant customer, a waiter and the kitchen.

• The customer represents the application making a request.
• The waiter represents the API carrying the request.
• The kitchen represents the server or service performing the operation.
• The prepared meal represents the response returned to the application.

The customer does not need to know how the kitchen is organized or how every dish is prepared. The menu explains what can be requested. The waiter delivers the order and brings back the result.

An API provides software with a similar, clearly defined communication channel.

A typical API operation follows these steps:

1. A client application sends a request to a specific address.
2. The API checks the request format and authorization.
3. The server performs the requested operation.
4. The result is returned in a defined data format.
5. The client displays the result or uses it in another operation.

What are API requests and responses?

The message an application sends to an API is called a request. The message returned by the API is called a response.

For example, a store application might request information about the product with an ID of 42:

GET /api/products/42

If the product exists, the API could return a JSON response like this:

{
  "id": 42,
  "name": "Wireless Keyboard",
  "price": 1499.90,
  "available": true
}

The application can then display the product’s name, price and availability in its user interface.

The important point is that the application does not connect directly to the database. It communicates through the API and can access only the operations and fields the API allows.

What is an API endpoint?

An endpoint is an address used to access a particular resource or operation within an API. The same API may provide different endpoints for users, products, orders and payments.

For example:

• /api/products may represent a list of products.
• /api/products/42 may represent one specific product.
• /api/orders may handle order-related operations.
• /api/users/me may return the current user’s profile.

The endpoint identifies the resource, while the HTTP method usually describes the intended operation.

HTTP methods commonly used by APIs

Web APIs commonly use the following HTTP methods:

• GET: Retrieves a resource or a collection of data.
• POST: Creates a new record or starts a new operation.
• PUT: Replaces or fully updates an existing resource.
• PATCH: Updates selected fields of an existing resource.
• DELETE: Deletes a resource.

For example, GET /api/products/42 may retrieve a product, while DELETE /api/products/42 may request its deletion. Whether the operation is allowed depends on the user’s permissions and the rules of the API.

What do HTTP status codes mean?

API responses usually contain an HTTP status code describing the result of the request.

• 200 OK: The request was completed successfully.
• 201 Created: A new resource was created successfully.
• 400 Bad Request: The request contains missing or invalid information.
• 401 Unauthorized: Authentication is required or the credentials are invalid.
• 403 Forbidden: The user is authenticated but does not have permission to perform the operation.
• 404 Not Found: The requested resource could not be found.
• 429 Too Many Requests: The client has exceeded the allowed request limit.
• 500 Internal Server Error: An unexpected error occurred on the server.

A well-designed application must handle error responses as carefully as successful responses.

API keys and authentication

Not every API is open to everyone. Many services need to identify the application or user making a request.

Common authentication methods include:

• API key: A unique access key assigned to an application.
• Bearer token: A token commonly used for authenticated user or application sessions.
• OAuth: A method that allows users to grant limited access without sharing their passwords with another application.
• Basic authentication: A simpler method based on a username and password that must be used over a secure connection.

API keys and access tokens should not be placed in public JavaScript files, shared repositories or other locations accessible to users. Sensitive credentials should normally remain on the server side.

What is a REST API?

REST is an architectural approach frequently used for web APIs. REST APIs commonly communicate over HTTP and return data in JSON format.

REST is not a programming language or a ready-made piece of software. It describes principles such as representing resources through addresses, using standard HTTP behavior and separating the client from the server.

Not every endpoint that returns JSON is a perfectly RESTful API. In everyday use, however, many HTTP-based services are described generally as REST APIs.

What other types of API are there?

REST is widely used, but it is not the only approach to building an API.

• SOAP: An XML-based protocol with a more formal and structured set of rules.
• GraphQL: Allows clients to request the specific fields they need.
• WebSocket: Supports a persistent, two-way connection between a client and server.
• RPC: Focuses on calling a function or procedure on a remote system.

The right approach depends on the project’s requirements, existing infrastructure, performance needs and the experience of the development team.

Where are APIs used?

APIs are used across almost every area of modern software. Many everyday services communicate with several APIs without making those connections visible to the user.

• Processing payments through a payment provider
• Tracking shipments and delivery status
• Displaying maps, locations and routes
• Retrieving weather information
• Sending emails and text messages
• Signing in through a social media account
• Accessing exchange rates and financial data
• Sending prompts to artificial intelligence models
• Connecting a mobile application to its server
• Synchronizing information between business systems

For example, restaurant software may send menu data to a mobile application, forward an order to a kitchen screen and retrieve a payment result from an external provider. The customer sees one interface, but several systems may be working together behind it.

What is the difference between an API and an SDK?

An API defines how a system can be accessed and how communication should take place. An SDK, or Software Development Kit, is a collection of tools designed to make development for a particular service or platform easier.

An SDK may include libraries, ready-made functions, documentation, sample projects and testing tools. It often provides a convenient way to work with the service’s API.

In short, an API defines the communication rules, while an SDK provides tools that help developers use those rules.

Are APIs and web services the same?

The terms are closely related, but they are not identical. A web service communicates over a network, usually through web technologies. The term API has a broader meaning and can also describe functions provided by a software library or an operating system.

Web services expose APIs, but not every API needs to be a web service.

What makes a good API?

An API must do more than simply work. Developers should be able to use it securely, consistently and efficiently.

A well-designed API generally:

• Uses consistent endpoint and field names.
• Provides clear and current documentation.
• Returns appropriate status codes for success and failure.
• Separates authentication from authorization.
• Validates submitted data on the server.
• Applies rate limits and protections against abuse.
• Manages version changes carefully.
• Avoids returning unnecessary or sensitive information.

API design is not only a technical concern. A good API reduces mistakes, shortens integration time and makes the intended behavior easier for other developers to understand.

Can artificial intelligence help with API development?

AI tools can help draft endpoints, prepare example requests, write documentation and interpret error messages. However, generated code still needs to be reviewed for security, authorization, validation and compatibility.

The developer remains responsible for protecting API keys, checking user permissions, filtering sensitive data and testing failure scenarios.

You can learn more about the benefits and risks of AI-assisted software development in our guide to vibe coding.

Frequently asked questions

Do you need programming knowledge to use an API?

You do not need advanced programming knowledge to understand what an API does. Integrating an API into an application, however, usually requires a basic understanding of programming, HTTP requests and data formats.

Are APIs free to use?

It depends on the provider. Some APIs are completely free, while others offer a limited free allowance. Commercial APIs may charge according to the number of requests, features used or amount of data processed.

Can an API key be shared?

API keys should generally be kept private. If a key becomes public, other people may use your account, consume your request allowance or create unexpected costs.

What is JSON?

JSON is a text-based data format commonly used to exchange information between systems. It is relatively easy for both humans and software to read. Many modern web APIs use JSON for requests and responses.

What happens if an API stops working?

Features that depend on the API may also become unavailable. Applications should therefore implement timeouts, appropriate retries, error logging and clear messages for users.

Conclusion

An API is an interface that allows different software systems to communicate through defined rules. APIs support everything from mobile applications and payment systems to shipment tracking and artificial intelligence services.

To understand the fundamentals, start with endpoints, requests, responses, HTTP methods, status codes and authentication. Once these concepts are clear, reading API documentation and following integration workflows becomes much easier.

Whether you are building a small application or a large software platform, APIs make it possible to combine existing services instead of developing every feature from the beginning.