Summary
Moodle is a leading open-source course management system (CMS) - a software package designed to help educators create quality online courses. Such e-learning systems are sometimes also called Learning Management Systems (LMS) or Virtual Learning Environments (VLE). One of the main advantages of Moodle over other systems is a strong grounding in social constructionist pedagogy.
(I must apologise to Tim for that last bit, as it came from the Moodle website - and no we still don't know what it means :-