is an RCS command ("Revision Control System"), used for source code control. i.e., it keeps track of older versions of files as you modify them over time. You can go back to previous versions easily should you accidently screw-up the most recent version.
Read up on RCS at: http://www.faqs.org/docs/Linux-HOWTO/CVS-RCS-HOWTO.html
Back in the "old days", it was "cp file file.bak". Then came SCCS ("Source Code Control System"). Then RCS. Then CVS (I think that one means "Concurent Versioning System"). CVS adds to RCS the ability of multiple people to be doing simultaneous editing, among other things. If it's just YOU using the system, CVS is overkill. RCS is just fine. There are other source code control systems out there as well. "subversion" being one such example you may have heard of.
Many people don't use source code control. For the life of me I can't figure out why. They often times come here asking "I accidently blew away this critical file, how do I get it back?"