WordPress is a free and open-source content management system based on PHP and MySQL. Features include a plugin architecture and a template system. It is most associated with blogging but supports other types of web content including more traditional mailing lists and forums, media galleries, and online stores.
WordPress powers over 30% of all websites and is by far the most used CMS on the planet. Backed by a huge community, this open source platform powers a multi-billion dollar economy with themes/plugins and custom software.