CVS = Concurrent Versioning System
In English: it's a system to keep track of changes made to code when there are multiple people working on the same project.
When you download a cvs package from a CVS server, you are downloading the absolute latest version of the software. And by that, I mean you are typically getting code that was merged into the project that day. The project is being actively developed, and you're downloading a daily snapshot.
This is great if you want to stay on the bleeding edge. It's not so great if you want stability. Someone may have just merged in code that has a bug. Someone may have checked in code that prevents a successful compile. If the developers have a strict checkin policy, then that lowers the chances, but people are human and sometimes make mistakes.
However, some projects will create "releases" inside CVS that you can check out. These are considered stable because the set of files won't change; they're frozen. With this route, you obviously aren't getting the latest-and-greatest.
My suggestion: unless you're a developer with the project, then you probably shouldn't mess with CVS unless it's the only way the project team offers the code (free Cedega is an example), or at least, stick to stable releases.