A REST API is a software design pattern that allows two different pieces of software to communicate with each other, enabling the integration of functionality from third-party services or other services within an application. It follows specific principles such as resource-based approach, uniform structure, statelessness, and hypermedia support. To interact with a REST API, developers use HTTP methods and headers to send requests and receive responses, which typically include an HTTP status code, requested content in a specific format, and additional information like query string parameters or authentication details. By adhering to these principles, REST APIs provide a scalable, flexible, and easy-to-use solution for communication between systems, allowing separation of concerns between client and server, and making it the standard for most web applications.