CVS, or Concurrent Version System, is one way to track changes made to any sort of file. Once an initial file is imported into the system, all changes to that file are checked when the file is "committed" back to the repository.
CVS allows for multiple people to be working on the same project, even the same files, at the same time, while carefully keeping track of what changed, and what stayed the same. CVS is used within the system to provide that level of power and control for you and your team across many projects.
ViewCVS: The web front-end to the CVS repository can be found here