An API (Application Programming Interface) defines a set of protocols and rules that allow applications to communicate with each other, enabling service integration and data exchange. You can think of an API as a contract that specifies the requests and responses between a consumer and a provider of information, such as a weather API that returns the temperature based on a postal code.

APIs serve as intermediaries between users (or clients) and web resources or services, allowing secure and controlled information sharing. They do not require knowledge of how resources are retrieved, which simplifies integration.

The REST architecture (Representational State Transfer) defines a style for API development. Unlike a protocol or standard, REST sets constraints that developers follow when building APIs. RESTful APIs transfer data over HTTP in formats like JSON, which both humans and machines can easily read.

The criteria for a RESTful API include a client-server architecture, stateless communication (no storage of information between requests), data caching to optimize interactions, and a uniform interface that allows resource manipulation. Compared to more complex protocols like SOAP, REST APIs remain simpler, faster, and more scalable, making them ideal for applications like IoT (Internet of Things) or mobile apps.