Moodle takes the principles behind Content Management System (CMS), used by many websites and blogs and applies them in an educational context. This helps teachers to share work and provide lessons online.

As it is an Open Source project, there are no fees to pay to install and use it, and there are plenty of add ons that can be installed for specific jobs.

There is plenty of help and information on the documentation site, as well as the complete text of the Using Moodle book that can be downloaded as a PDF and printed.

There is a demo site if you want to get your hands dirty and have a go before downloading.