I believe it's short for concurrent version system, but it is a way for developers to all be working on code and all post updates at different times. Sometimes, if you don't use a system like this, what happens is a splintering of code - one project makes some changes, and another project makes some changes, but they don't work right together.
CVS is where the CURRENT code is stored. It can change as little as once every few days to 50 times in a day. CVS code usually isn't supported well because it's not meant to be in the hands of end users who don't know what they're doing. A client using CVS code is usually helping with development or bugchasing, or NEEDs the newest features.
If you still need more information, read up on
cvs for windows. Most linux users already know what to do with it, and how to access it, as it comes built into many distributions.