PHP is basically a programming language for the web... with it, you can create dynamic pages and good stuff like that.
For instance, the home page of xibase, as well as this bulletin board, is powered by PHP and MySQL (a relational database management system)
Other programming languages for the web include Perl and ASP, but PHP owns them
Since PHP is open source it can run on virtually any web server, and that's why it's the future of the web.