Moodle is a Course Management System (CMS), also known as a Learning Management System (LMS) or a Virtual Learning Environment (VLE). It is a Free web application that educators can use to create effective online learning sites.

Moodle is an Open Source software (under the GNU Public License). Moodle can be installed on any computer that can run PHP, and can support an SQL type database (for example MySQL). It can be run on Windows and Mac operating systems and many flavors of linux (for example Red Hat or Debian GNU). 

See also

What does a teacher need to know? Learn about the features in a Moodle course.
Moodle Demonstration Site. Use a Moodle site as a teacher, administrator or student.
Moodle_manuals Using Moodle book for those who must have an Adobe document to read or print.

Website http://moodle.org/