WordPress is a free, open-source content management system (CMS) based on PHP and MySQL. It has a core software that is readily and highly extensible by adding plugins. The look and functionality of a WordPress website is built on a system known as a theme, a software extension somewhat like a plugin. Themes can be created, installed, and managed without altering the content or structure of the site. 25% of the top 10 million websites are said to be built on WordPress. It is also said to be the easiest and therefore most popular system for managing a website or blog, employed in more than 60 million websites. More on the WordPress website.