According to Wikipedia:
WordPress is an open source blog publishing application powered by PHP and MySQL which can also be used for basic content management. It has many features including a user-friendly workflow, a rich plugin architecture, and an advanced templating system.
Users may choose from a variety of either free or premium themes that enables them to alter the appearance and functionality of their website without altering the core code or the site content. WordPress’ massive library of 50,000+ plugins provides further customizability, as do the addition of widgets and navigation bars. While some plugins are attached to the basic WordPress package, others developed by third-parties can be installed only with purchase.
As of January 2018, the popular website management system supports more than 60 million websites and 29.3% of the top 10 million websites. Since merging with WordPress MU (previously a fork of WordPress), WordPress 3 now integrates a multi-user and multi-blogging feature that provides users a centralized platform to administer their websites or blogs.
It was first released in May 2003 by Matt Mullenweg as a fork of b2/cafelog, the precursor to WordPress.
WordPress has been used during phase 2 of the Open Government Dialogue as well as for the Department of Defense Web 2.0 Guidance Forum.