Request and Response Model
HTTP is simple: a client-most conspicuously a web browser-sends a request for some resource to a HTTP server, and the server sends back a response. The HTTP response carries the resource-the HTML document or image or whatever-as its payload back to the client. This simple request-response model is shown in Figure B-2.
Figure B-2. Browser makes a request and the HTTP server responds
The term HTTP server is the correct description for what is more commonly called a web server. Technically, a web browser is an example of a user agent. Other user agents include proxy servers, applications that can provide services such as caching of responses, and access control.