A client application (like a mobile app, web browser, or another server) initiates a request to the API to perform an action. This request is sent to a specific URL and includes an HTTP method, headers with metadata, and sometimes a request body with data.

The server receives and validates the request, confirming that the client is authenticated and authorized to perform the requested action. It then processes the request, which could involve retrieving data from a database, creating a new resource, or updating an existing one.

After processing the request, the server sends a response back to the client. This response includes an HTTP status code indicating the outcome (for example, 200 OK for success, 404 Not Found if the resource doesn't exist, 401 Unauthorized for security issues), along with the requested data in the response body.

The data transferred between the client and server is a "representation" of the state of the resource. JSON (JavaScript Object Notation) is the most common format for this data because it is lightweight, human-readable, and easy for programming languages to parse.

The power and scalability of REST come from a set of architectural constraints that guide its design. A system that adheres to these constraints is considered "RESTful."

A resource is a fundamental concept in REST. It is an object with a type, associated data, relationships to other resources, and a set of methods that operate on it. For example, in an e-commerce API, a "product" or a "customer" would be a resource.

Each resource is uniquely identified by a URI. A well designed API uses clean, descriptive, and consistent URIs to identify its resources. For example, /users might identify a collection of users, and /users/123 would identify a specific user with the ID 123.

A client does not interact with the resource directly but with a representation of that resource. The most common representation format is JSON, but others like XML or HTML can also be used. This provides a flexible way to represent the state of a resource.

Every request from a client to the server must contain all the information the server needs to understand and process the request. The server does not store any client context or session state between requests. This constraint makes REST APIs highly scalable because any server can handle any request.

The client and server are separated, with the API acting as the interface between them. This separation of concerns allows the client-side application and the server-side application to evolve independently.

REST requires a consistent, uniform interface between the client and server. This is achieved through the use of standard HTTP methods, URIs for resource identification, and hypermedia as the engine of application state (HATEOAS).

Responses from the server should be defined as cacheable or non-cacheable. When a response is cacheable, a client or intermediary can reuse that response for subsequent, identical requests, which can significantly improve performance and reduce server load.

A client connected to a REST API cannot ordinarily tell whether it is connected directly to the end server or to an intermediary along the way. Intermediary servers, such as API gateways or load balancers, can be introduced to the architecture to improve scalability, security, and performance.

A common use case is a mobile weather app fetching the current weather for a specific location from a public weather API.

The client application makes an HTTP GET request to the API endpoint, including the location and an API key for authentication.

The server processes the request, retrieves the weather data for the specified location, and returns a JSON response.

In a microservices architecture, a "product" service might need to get an updated price from an "inventory" service before displaying it on an e-commerce site. 

The product service makes an internal HTTP GET request to the inventory service's API endpoint for a specific product ID.

The inventory service looks up the current price and returns a simple JSON response.