WordPress [WP] is an open source Content Management System for building apps, websites, and blogs. WP is written in PHP and uses MySQL as the data store for the user content and configuration. It has a rich ecosystem of plugins and themes and enjoys a vibrant open source community, good documentation, and low barriers to entry. Usability and developer documentation can be found in the WP Codex.
A part of WordPress that makes it different from most other CMS products is its Event Driven Programming. This is a different way of programming and logic representation then the MVC (Model View Controller) architecture which is used by most of the CMS systems. WordPress uses the concepts of Actions and Filters. They form a queue of events that allow plugins and themes to insert, modify or even remove parts of the final web application page and/or parts. A similar concept is JIT or Just-In-Time compiling.