Welcome to Moodle!
Moodle is a course management system (CMS) - a free, Open Source software package designed using sound pedagogical principles, to help educators create effective online learning communities. You can download and use it on any computer you have handy (including webhosts), yet it can scale from a single-teacher site to a University with 200,000 students. This site itself is created using Moodle, so check out the Moodle Demonstration Courses or read the latest Moodle Buzz.
Moodle has a large and diverse user community with over 330,000 registered users on this site alone, speaking over 70 languages in 196 countries (we have more statistics here). The best place to start is Using Moodle, which is where the main international discussions are held in English, but we have a variety of groups discussing other topics and in other languages.
We have a long list of developers who contribute towards the development of Moodle. Start with our developer information as well as the roadmap, the coding guide and CVS guide to access our source code. Most discussion can be found in Using Moodle, but we also have a Tracker where you should report general bugs, feature requests, fixes and other issues. Sensitive security problems should be posted to the Moodle Security site, so we can deal with them appropriately.
Moodle is available in a variety of download packages with different levels of stability, as well as via CVS. A number of additional modules/plugins and language packs are also available.