And SCCS came. He made RCS, CVS, SVN

SCCS and his children keep track of versions and revisions
They save all the revisions, by storing diffs
The new ideas of "checkout" and "commit" appear
Programmers can use symbolic names («tags»)
The tools automatically generate version numbers (e.g.: 1.432)
Every commit is augmented by a log message

CVS, quite widespread, is mainly just RCS over the network
Its own files live in a centralized repository
Several uses can concurrently access the repository to read or write
The network protocol is simple
Conflicts are handled, though with some limitation

What's wrong with CVS:
Revisions are only maintained within each individual file
If you rename a file you loose its history
If you split or merge files, you loose their history
Committing several files is not an atomic operation