From the GNU site:
The Revision Control System (RCS) manages multiple revisions of files. RCS automates the storing, retrieval, logging, identification, and merging of revisions. RCS is useful for text that is revised frequently, including source code, programs, documentation, graphics, papers, and form letters.
Currently RCS isn't used much anymore for "regular" revision control but comes in quite handy when you need to keep versions but have no need for an extensive external repository.