Just out of curiosity, why do you need to know what CVS is? A few things to note, if you are considering starting a repository, I've found subversion easy to use and as they say it is an updated CVS style repository. There are tons of revision control systems on both the open source and commercially supported sides of the spectrum, one thing that makes CVS good, is the fact that it, like subversion, is open source, although I've heard that active development on the original CVS ceased a couple of years ago and openCVS took over for them. From my experience with CVS, it seems a bit outdated and didn't provide some of the functionality I thought should be inherent to a Versioning System. Okay, since you asked what CVS was I'll put it this way (I don't know what you do and do not know, so I'll start at the beginning) A repository is a type of programs that serves to maintain a large (sometimes very large) base of source code. The repositories are called Version Control Systems or Source Code Control Systems. These make it easy for developers to work on the same project and all have the same base of code to work off of and it also makes it easier for them to roll back changes that adversly affect the project they are working on. CVS - Concurrent Versioning System is one of if not the oldest SCCS and is still quite widely used for projects of many sizes (lots of opensource projects use CVS.) However, recently there have been newer, more up to date systems released, some of which are open source such as subversion, but there are also several that are in the commercially supported sector, such as Source Safe from M$, Rational from IBM, Perforce, and many more. These products have varying prices based upon their protection measures and ability to handle larger and larger bases of code. Rational, from what I've heard, never used it personally, is the most advanced, which is not a suprise, considering its $50,000+ licensing. I have used Perforce, Subversion and CVS, and if I had to choose a favorite from the three, Perforce wins hands down, it's easy to use, follow, and provides you with a stable base for your code.

Not to sure if any of that was what you were after, so I'll conclude with a few reminders. 1) CVS is used to store source code to large projects and provide them with versioning support. 2) There are newer and better solutions available in both the open source and closed source arenas. 3) Source Code Control Systems can be both fun and entertaining, not really but it would be pretty cool if they were....