The Concurrent Versioning System (CVS) is simply a method of storing files on a server that helps coordinate many people working on those files. It keeps track of every version of every file in a software project.

The "master copy" of Moodle is stored on a CVS server at - you can browse it via the web.

To "check out" files from a CVS server means to copy the very latest versions to your own computer - anyone can check out files using an anonymous account.

To "check in" files means to update the master version with new code - only registered Moodle developers can check files in.

» Moodle Terms