What principles and characteristics must an API adhere to in order to be considered RESTful?

A RESTful API must adhere to the following principles and characteristics:

• Client-server architecture: separation of client and server logic.
• Stateless: each request from the client to the server must contain all the necessary information; the server does not store state between requests.
• Cacheability: responses should explicitly indicate whether they can be cached to improve performance.
• Uniform interface: use standard HTTP methods (GET, POST, PUT, DELETE, etc.) and clearly define resources via URIs.
• Layered system: architecture can consist of multiple layers, such as proxies, load balancers.
• Code on demand (optional): the server can send executable code to the client (e.g., scripts).

Example: for a "user" resource, the URI might be /users/123, where GET retrieves user data, POST creates a new user, PUT updates, and DELETE deletes.