Apache is a widely used web server available for many platforms, and provides a consistant, scalable, and secure mechanism with which to serve web pages. It is the most popular HTTP server on the web.

The name, like everything else in the computing industry, is a pun (or terrible joke, either way). Apache came out of the ashes of a pile of other servlet components all held together by patches, hotfixes, and little hacks. Thus it was a "patchy" server.

(Okay, you can stop groaning now. It was terrible when i first heard that too)