WordPress is a free and open-source content management system (CMS) 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 is designed from the ground up to be highly customizable. Its application programming interface (API) covers everything from its database to theme modifications and customizations. By keeping everything standardized, developers can focus on building functionality on top of WordPress, knowing that it will work on all installations that are kept up-to-date.