A REST API (also known as RESTful API) is an application programming interface (API or web API) that conforms to the constraints of REST architectural style and allows for interaction with RESTful web services. REST stands for representational state transfer and was created by computer scientist Roy Fielding.
An API is a set of definitions and protocols for building and integrating application software.
Any web service that obeys the REST constraints is informally described as RESTful. Such a web service must provide its Web resources in a textual representation and allow them to be read and modified with a stateless protocol and a predefined set of operations.
REST is a set of architectural constraints, not a protocol or a standard. API developers can implement REST in a variety of ways.
When a client request is made via a RESTful API, it transfers a representation of the state of the resource to the requester or endpoint. This information, or representation, is delivered in one of several formats via HTTP: JSON (Javascript Object Notation), HTML, XLT, Python, PHP, or plain text.
The most common protocol for requests and responses is HTTP. It provides operations (HTTP methods) such as GET, POST, PUT, and DELETE. By using a stateless protocol and standard operations, RESTful systems aim for fast performance, reliability, and the ability to grow by reusing components that can be managed and updated without affecting the system as a whole, even while it is running.