API (from English Application Programming Interface) is a set of rules, protocols, and tools that enable different software systems to communicate with each other. An API defines how one program can request data or functionality from another without knowing the details of its internal implementation.
Principle of Operation
An API acts as an intermediary between a client (application, service, or user) and a server or another system that provides resources. When a program sends a request to an API, the interface processes the request and returns a response in a standardized format — most commonly JSON or XML.
For example, a mobile app may send a request to a server’s API to retrieve a list of users. The server processes the request, extracts data from the database, and returns it as a structured response.
There are several types of APIs:
- REST API — the most common type, using HTTP requests;
- SOAP API — based on XML and strict specifications;
- GraphQL — a flexible interface allowing clients to request only the data they need;
- WebSocket API — provides real-time, two-way communication.
Applications
APIs are used in virtually all modern IT systems:
- for integrating services (e.g., a CRM with a payment gateway);
- in cloud infrastructures to manage resources and virtual machines;
- in web development to connect external data and functionality;
- in mobile applications, where the client interacts with the server via REST API;
- in DevOps, for process automation and data exchange between tools.
Example: a map on a website is loaded using the Google Maps API, which gives developers access to display and geolocation features.
Advantages
Using APIs provides multiple benefits:
- standardization and unification of interactions between systems;
- faster development through reuse of existing functions;
- scalability and easy integration of new services;
- security, since there is no need for direct access to a system’s internal logic.
Example of Use
A company can use a payment service API to accept online payments on its website. The web application sends an API request with order details, and in response receives a transaction confirmation — without handling card data directly.