Moodle is an Open Source Course Management System (CMS), also known as a Learning Management System (LMS) or a Virtual Learning Environment (VLE). It has become very popular among educators around the world as a tool for creating online dynamic web sites for their students. To work, it needs to be installed on a web server somewhere, either on one of your own computers or one at a web hosting company.
The focus of the Moodle project is always on giving educators the best tools to manage and promote learning, but there are many ways to use Moodle. For example, Moodle has features that allow it to scale to hundreds of thousands of students or it can be used in an elementary or high school. Many institutions use it as their platform to conduct fully online courses, while some use it simply to augment face-to-face courses a.k.a. blended learning. Many users love to use the variety of activity modules, such as forums, wikis, and databases, to build richly collaborative communities of learning around their subject matter. Others prefer to use Moodle as a way to deliver content to students and assess learning using assignments or quizzes.
The word Moodle was originally an acronym for Modular Object-Oriented Dynamic Learning Environment, which is mostly useful to programmers and education theorists. It's also a verb that describes the process of lazily meandering through something, doing things as it occurs to you to do them, an enjoyable tinkering that often leads to insight and creativity. As such it applies both to the way Moodle was developed, and to the way a student or teacher might approach studying or teaching an online course.
There are even Moodlemoots which are conferences held all over the world that can be attended in person or virtually. There's are two this month in Orlando and Madrid, and in New Zealand in April, 2010!
Moodle also offers Moodle Docs which is similar to Google Docs. MediaWiki is the technology that drives Moodle Docs.
If you use Moodle, then you are a Moodler.
Sources:
www.moodle.org
www.iste.org
Language Learning through Multimodal Communication in VEC3D (a study in a PDF)