Moodle is a Learning Management System (LMS) that allows instructors to expand the boundaries of their classroom outside the traditional face-to-face classroom. It is a web-based system where course materials (such as documents, assignments, audio and video files) can be posted online, making them more convenient for students at all times of the day. This reduces the use of paper and other resources, and allows for development of activities that incorporate a wide variety of delivery modes. In addition, Moodle easily allows forums and discussion boards, which can expand conversations outside of class into a more in-depth and dynamic interaction.
(FYI for geeks: The word Moodle is an acronym for Modular Object-Oriented Dynamic Learning Environment!)