A REST API (Representational State Transfer Application Programming Interface) is a standardized architectural approach for enabling communication between software systems over the web, using HTTP as the underlying transport protocol and treating all accessible resources as uniquely addressable entities.
The term "Representational State Transfer" refers to the way data is handled: when a client makes a request, it receives a representation of the resource's current state rather than direct access to the resource itself. This representation is typically formatted as JSON or XML. The client can then use that representation to understand or modify the resource through subsequent requests.
REST APIs are built around a small set of standard HTTP methods that each carry a specific meaning. A GET request retrieves data, a POST request creates a new resource, a PUT or PATCH request updates an existing one, and a DELETE request removes it. This consistent use of HTTP verbs makes REST APIs relatively intuitive to understand and integrate with, regardless of the underlying technology stack.
One of the defining characteristics of a REST API is that it is stateless. This means the server does not retain any information about previous requests from the same client. Every request must contain all the information the server needs to process it, including authentication credentials if required. This constraint simplifies server infrastructure and makes REST APIs easier to scale horizontally across multiple servers.
REST APIs have become the dominant pattern for exposing functionality on the web because they align naturally with how HTTP already works. A web browser retrieving a page and a mobile app fetching user data are both performing actions that follow the same fundamental logic. This alignment makes REST APIs accessible to a broad range of clients, from browsers and native applications to server-side services and automated scripts.
In modern software development, REST APIs serve as the connective layer between services, systems, and user-facing products. When a payment provider, a mapping service, or a messaging platform exposes its functionality externally, it almost always does so through a REST API.