HTTP

Short for HyperText Transfer Protocol, HTTP is the underlying protocol used to transfer data over 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.

This is why all Web site addresses begin with "http://".