- An API defines how one software component can request data or actions from another.
- Web APIs commonly use HTTP methods, URLs, status codes and structured formats such as JSON.
- Authentication, authorization, versioning, rate limits and documentation are central to reliable API design.
An application programming interface, or API, is a defined way for software components to interact. It lets a caller use capabilities without needing to know every internal implementation detail.
APIs exist inside programming libraries, operating systems and distributed systems. Web APIs are especially visible because they connect applications across networks.
A simple API example
Imagine a weather application that needs a forecast. Instead of operating weather stations itself, it sends a request to a weather service API. The API returns structured data that the application can display.
The API contract describes which endpoint to call, which parameters are accepted, what authentication is required and what the response looks like.
Requests and responses
HTTP APIs expose endpoints identified by URLs. Clients send methods such as GET to retrieve information, POST to create something, PUT or PATCH to update resources, and DELETE to remove them where the API supports those operations.
Responses include a status code and often a JSON body. Codes in the 200 range generally indicate success, while 400- and 500-range responses describe client and server errors.
What REST means
REST is an architectural style commonly associated with resource-oriented HTTP APIs. A REST-style design might expose /articles/123 as a resource and use HTTP semantics to operate on it.
Not every HTTP API follows REST strictly. GraphQL, gRPC and event-driven interfaces offer different trade-offs for querying data, typed service communication and asynchronous integration.
Authentication and authorization
Public APIs may still need API keys to identify callers and enforce quotas. User-facing systems often use token-based mechanisms and standards such as OAuth 2.0.
Authentication establishes identity; authorization decides what that identity may do. Treating those as separate concerns is important for secure API design.
Why good API design matters
An API becomes a dependency for other software. Breaking changes can therefore affect many clients. Clear documentation, predictable errors, versioning policies and backward compatibility reduce integration risk.
Rate limits and observability protect services from accidental overload and help operators understand latency, errors and usage patterns.
APIs in modern software
Mobile apps, payment systems, cloud platforms and microservices all rely heavily on APIs. Infrastructure platforms also expose APIs, which is why tools can create servers, networks and Kubernetes resources programmatically.
The API is often the durable product boundary even when the implementation behind it changes completely.
Understand the system behind the headline.
The useful question is not only what a technology or market concept is, but what problem it solves, which incentives shape it, what trade-offs it introduces and when those trade-offs matter.
Questions readers often ask.
What is the difference between an API and a website?
A website is designed primarily for people, while an API exposes structured operations or data for software clients. A product can provide both.
Is every API a REST API?
No. REST is one approach. APIs can also use GraphQL, gRPC, library interfaces, message protocols and many other designs.
What is an API key?
An API key is a credential used to identify or authorize an application calling an API. It should be protected like other secrets.