WordPress is a web application that allows you to create quick and easy news items, blog posts, event pages and much more.
It was originally developed as a blogging tool, but because it is open source a large developer community allowed wordpress to evolve with the addition of plugins and different website themes. It is estimated that 1 in 4 websites are powered by wordpress, and there are currently over 55 million ‘official’ wordpress sites.
You can find further information below.
Official WordPress Site
www.wordpress.com
Technical Information and Open Source
www.wordpress.org
WordPress Stats
http://en.wordpress.com/stats/