This is an extract of what wikipedia.org has on WordPress:
“WordPress is a free and open source blogging tool and content management system (CMS) powered by 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 powers 22% of all new websites. WordPress is currently the most popular CMS in use on the Internet.”
I guess it says it all!