WordPress is an open source CMS (Content Management System), often used as a blog publishing application powered by PHP and MySQL. It has many features including a plugin architecture and a templating system. Used by over 12% of the 1,000,000 biggest websites, WordPress is the most popular CMS in use today.
WordPress is a free open source content management system. It uses templates, of which there are thousands in existence.
According to W3Techs, almost 55% of the 1 million most visited websites that are run on a content management system (CMS) are run on WordPress. WordPress is a darn fine CMS and is stable and easy to use,
Source: Smashing magazine