I found this handy little tutorial about CVS:
Understanding CVS: A brief introduction to the concepts of CVS

Nice pictures, clear explanation. NB: CVS is a revision control system used when multiple software developers need to collaborate on a project, it maintains consistency.

Edit: I changed the title of this post due to some...strong opinions about CVS, which is a legacy system, and SVN is pretty much it's successor. I just thought the explanation of the concepts was cute.