If you want to know all the gory details, you will want to visit the CVS main web page.

For the impatient, the following is probably about all you need to know:

cvs checkout -c: Will list the modules available for checkout
cvs checkout < module name >: Will checkout the named module
cvs co < module name >: Same thing
cvs update: Updates your local archive so it is in sync with the repository -- your local updates are left intact. Tries to merge upstream updates into your local updates. You will see the following tags when it is updating your local repository: C means conflict, U means update, P means patched, and M means modified.
cvs up: Same thing
cvs update < file name >: Same thing but for just the named file(s)/directory(s).
cvs commit: Will check in all your work.
cvs add < file name >: Adds the named file/directory into CVS
cvs remove < file name >: Removes the named file/directory from the upstream repository.
cvs rm < file name >: Same thing
cvs log < file name >