A REST (Representational State Transfer) API is a set of conventions and protocols that allows communication between systems over HTTP. This architectural style is widely used to build web services that facilitate the integration of different applications. REST APIs allow developers to access resources and data easily using methods such as GET, POST, PUT, and DELETE. They are fundamental for creating scalable and flexible applications, as they allow disparate systems to interact in a standardized way. Furthermore, their JSON-based structure facilitates data transfer and interoperability between different languages and platforms, making service integration smoother and more efficient.