Short for HyperText Transfer Protocol, the underlying protocol used by the World Wide Web. HTTP defines how messages are formatted and transmitted, and what actions Web servers and browsers should take in response to various commands. For example, when you enter a URL in your browser, this actually sends an HTTP command to the Web server directing it to fetch and transmit the requested Web page.

HTTP differs from other TCP-based protocols such as FTP, in that connections are usually terminated once a particular request (or related series of requests) has been completed. This design makes HTTP ideal for the World Wide Web, where pages regularly link to pages on other servers. It can occasionally pose problems for Web designers, as the lack of a persistent connection necessitates alternative methods of maintaining users' "state". Many of these methods involve the use of "cookies".

HTTP is a request/response protocol between clients and servers. An HTTP client, such as a web browser, typically initiates a request by establishing a TCP/IP connection to a particular port on a remote host (port :80 by default). An HTTP server listening on that port waits for the client to send a request string, such as "GET / HTTP/1.1" (which would request the default page of that web server), optionally followed by an email-like MIME message which has a number of informational header strings that describe aspects of the request, followed by an optional body of arbitrary data. Upon receiving the request string (and message, if any), the server sends back a response string, such as "200 OK", and a message of its own, the body of which is perhaps the requested file, an error message, or some other information