Hypertext Transfer Protocol (HTTP)
Hypertext Transfer Protocol (HTTP) is an application layer protocol used by web browsers and web servers to transfer files, such as text and graphic files. It is a client-server protocol; a client (usually a web browser) requests a resource (a web page) from a web server. The web server responds with the requested web page.
Consider the following example:
The client wants to access http://google.com and points his browser to the URL http://google.com (this is an example of an HTTP Request message). The web server hosting http://google.com receives the request and responds with the content of the web page (an HTTP response message).
Web servers usually use a well-known TCP port 80. If the port is not specified in a URL, browsers will use this port when sending HTTP request. For example, you will get the same result when requesting http://google.com and http://google.com:80.