WordPress is an open source Content Management System (CMS) which is used to build and manage websites. WordPress is the most popular CMS on the internet by a country mile, powering about half of all CMS websites at time of writing and about a quarter of all websites on the internet.
WordPress started life as a platform for blogging but has evolved over the years to be suitable for most types of websites. The interface can be used without coding knowledge making it popular for beginners and developers who want to empower their clients to manage their own website.
Another large factor in the popularity of WordPress is it’s flexibility, mostly due to the core’s plugin and theming systems. The plugin system makes it easy to extend the core functionality without modifying the core code. In a similar manner, the theming system makes it easy to change the website’s layout and aesthetics. There are now thousands of free and premium WordPress plugins and themes available. Many of these are located at the wordpress.org plugin repository and theme repository respectively.
WordPress is developed by it’s own community, but is strongly associated with the company Automattic, which employs many of WordPress’ core developers.
WordPress is built upon the PHP server scripting language and the MySQL querying language. WordPress uses MySQL as a datastore for user content and configuration. The PHP wrangles the content data into a HTML webpage with all the necessary assets.
wordpress.com vs wordpress.org
You can use WordPress by signing up for Automattic’s wordpress.com service and hosting your website on their servers, or you can download the WordPress software from wordpress.org and host your website on a server under your control. The first option is easy but you cannot edit any site code. You can only make changes through the WordPress interface. The second option requires more work but gives you flexibility to do whatever you like with your website code. If you are a StackOverflow user, you probably will be going with the second option.
WordPress is open source software meaning it is free to use and anyone can view the source code and contribute to it. Potential contributors can get started by reading the Contribution page of the WordPress codex..
Bugs can be reported by submitting a bug on the WordPress ticket tracker.
WordPress is officially documented in the WordPress Codex at WordPress.org. Developers working with WordPress will be particularly interested in the Developer Codex section and Developer Reference section of wordpress.org.