PHP is a RecursiveAcronym for PHP: Hypertext Preprocessor. (It should be noted that all RecursiveAcronyms? are also Acronyms, which has just scored this page a link from the Acronym page :)

PHP is a widely-used general-purpose scripting language that is especially suited for Web development and can be embedded into HTML.

This Wiki is written in PHP. Other cool things written in PHP include Cacti, most WebMail clients and The Eight Ball

PHP.net have a great introductory tutorial. The online manual is very useful.

PHP is a member of the LAMP 'foursome' of Linux, Apache, MySql? and PHP which is very commonly used for hosting small data driven web sites.