RCS keywords are dollar-sign delimited strings that a revision control system like CVS interprets and automatically expands if a file is committed. These keywords allow for example to insert into a file automatically the file name and revision number and date as well as a file history log.

Some of the usefull RCS keywords:

$Author: $ - The user responsible for a change.
$Date: $ - Date and time of the change, in GMT.
$Header: $ - A collection of information: full path to the RCS file, revision number, date/time of last change (GMT), author, state, and locker.
$Id: $ - Like $Header: $, but without the path to the RCS file.
$Log: $ - Inserts the cvs log message. This behaves differently than other tags in that it just inserts new information after the keyword. It doesn't replace the keyword or modify existing text.
$RCSfile: $ - Name of the RCS file in the repository
$Revision: $ - Revision number