Every time you make changes to a file that's stored in CVS, a new revision is created.
Each revision includes the date of the change, the name of the user who made the change, and a log message that describes the change. You can retrieve arbitrary revisions of a file for inspection at any time. You can use symbolic names, called tags, to mark a particular revision for easy reference.
A revision is denoted by a sequence of numbers and dots. It's analogous to the standard numbering scheme used for versions of software. For example, a file called foo.c might have had three changes over the last few days. The first revision would be numbered 1.1, the second would be 1.2, and the third 1.3. CVS automatically assigns the numbers and uses them internally. You'll have to use these numbers on many occasions.
The changes in foo.c are cumulative, so revision 1.3 contains all the changes made between 1.1 and 1.2, as well as the changes made between 1.2 and 1.3.