RCS et al can put special strings for identification into your source code. When you place the marker $Id$ into your code (for instance inside a comment) RCS will replace this marker with a string of the form:

$Id: filename revision date time author state $
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When you use RCS/CVS/Subversion you check out files from a repository, work on them, and check them back in. Each time you check the files back in the $Id$ string is automatically updated. If you introduce a bug you can roll back the changes to an earlier version or do a diff between the old working version and the new buggy version. The concept is called source control and RCS/CVS/Subversion are implementations of this.