API

An application programming interface (API) is a set of rules and specifications that software components can follow to communicate with each other. It defines the methods and data formats that applications can use to request services from libraries, operating systems, or remote servers, without needing to know the internal implementation.

APIs can be categorized by context. Web APIs enable communication over a network via HTTP or HTTPS; library or framework APIs expose functions within a programming language; operating system APIs provide access to system resources; and hardware or service APIs allow interaction with devices or cloud services. Data formats commonly used in APIs include JSON and XML, and architectural styles include REST, SOAP, GraphQL, and gRPC.

In a typical web API, a client uses endpoints that correspond to resources or actions and supports HTTP methods such as GET, POST, PUT, and DELETE. Data is exchanged in requests and responses, and the API defines how parameters are provided, what data is returned, and how errors are conveyed. APIs are often designed to be stateless, scalable, and versioned.

In practice, a client sends a request to an API endpoint with the method, path, and parameters, often including authentication credentials such as an API key or OAuth token. The service authenticates, processes the request, and returns a response with data and a status code. Good API design emphasizes clear documentation, versioning, and stable contracts, along with security and reliability considerations.

APIs enable modularity and interoperability, allowing systems to integrate, automate tasks, and reuse functionality across different applications and platforms. They are foundational to modern software development and digital ecosystems.