WordPress is a free and open source content management system (CMS) based on PHP and MySQL. It is the most widely-used CMS software in the world and it powers more than 29% of the top 10 million websites (https://w3techs.com/ as of Feb/2018), giving it an estimated 60% market share of all sites using a CMS.
WordPress started as just a blogging system in 2003, but has evolved to be used as full content management system and so much more through the thousands of plugins and widgets and themes.
WordPress is licensed under the General Public License (GPLv2 or later).
Refer this article for a list of the features that come as standard with WordPress.