According to the WordPress Codex, "WordPress powers nearly a quarter of new sites today, is the content management system (CMS) of choice for more than two thirds of the top million sites making it the most popular on the web, and is trusted by content publishers both large and small including CNN and the NY Times. With more than 50 million sites globally and eight years of history."
The roots go back to 2001, but it made its first appearance in 2003. You can view it's history in the Codex on this page.
This is the official website responsible for all communication and resources related to WordPress. From this site you can download a copy, as well as search for free plugins and free themes. Access to the codex and and the WordPress Forum can be found here as well.
The WordPress Codex
We've mentioned it several times. This is the location of all documentation pertaining to WordPress. It is intended to be an encyclopedia of all WordPress knowledge. It is an "open wiki" which means that it is written by contributors. You can add to or modify wiki articles. You can read more about it here. If you want to contribute, start with this page.
Like most software, WordPress is distributed under a license, which means there are certain things that you are legally permitted (and not permitted) to do with WordPress software and source code. WordPress is distributed under a license called the GNU General Public License, a very popular license in the open source industry. If that doesn't ring a bell, read on.
About the GPL
The GNU General Public License, or GPL, is an open source license. Open source doesn't just mean that you can view the source code — it has political and philosophical implications as well. Open source, or "Free Software", means you are free to modify and redistribute the source code under certain conditions. Free doesn't refer to the price, it refers to freedom. The difference between the two meanings of free is often characterized as "Free as in speech vs. free as in beer." The GPL is free as in speech.