How does CVS work?

A CVS repository exists on a server somewhere.

Users check out a working copy onto their local system.

After changes are made to files in the local working copy, a user will commit those changes to the server repository.

Before starting work on the files, and periodically during work, a user will update their local working copy with changes that others have made to the repository.

If two people change the exact lines in a file independently, the last one to try to commit the file will receive a notice of a conflict and will have to resolve the conflict before they can commit the file.