Originally posted by Bri_Dog
So it's kinda keeping track of what people have done to the code?
Exactly.
You can see what changes were made, by whom, and at what time, at any step along the development path. You can go back 5 revisions if you need to grab code that was removed, for example. You can also have multiple branches (although this is a PITA most of the time), and merge them, etc.
It's an excellent way for a team to work on a common source base, especially when the code is large (like FS2) and the developers are scattered about (like us

)
CVS is only one of a number of version control systems, some free, some very expensive. CVS is popular amongst open source developers because it's free, it's supported on a variety of platforms, it can be used over the internet (with various levels of security), and it works.
See
http://www.cvshome.org for more info, and to download it (if you think you need it).