According to Wikipedia,
WordPress is a free and open source blogging tool and a dynamic content management system (CMS) based on PHP and MySQL. It has many features including a plug-in architecture and a template system. WordPress is used by over 14.7% of Alexa Internet’s “top 1 million” websites and as of August 2011 manages 22% of all new websites. WordPress is currently the most popular CMS in use on the Internet.
It was first released on May 27, 2003, by founders Matt Mullenweg and Mike Little as a fork of b2/cafelog. As of December 2011, version 3.0 had been downloaded over 65 million times.
WordPress is free. It’s for blogging. It’s for websites. It’s for photo blogs. It’s for video blogs. WordPress is for, well, almost anything and almost anyone! It’s for web beginner noobs, it’s for authors, it’s for visual artists, it’s for publishers, it’s for musicians, the list goes on and on.