CVS - Concurrent Versions System
Documentation
CVS Best Practices is a document that provides recommended guidelines to follow when using CVS. (Another source)
Essential CVS is a web-based wiki version of the reference from O'Reilly that is hosted on their O'Reilly Commons site.
Open Source Development With CVS, informally known as "the CVS book", is an open source book with the goal of teaching the effective use of CVS.
Version Management with CVS (for CVS 1.10.8), by Per Cederqvist
Some useful sections:
Branching/merging:
Branch and merge examples from Cal Poly, San Luis Obispo
Branching and merging in CVS, from the University of Wisconsin
Cheat sheets
CVS Intro and Cheat Sheet from Mu2e (muon-to-electron-conversion experiment) at Fermilab, a part of the U.S. Departement of Energy
A quick and dirty guide to CVS, from the Department of Computer Science at the University of Massachusetts, Boston
CVS Cheat-sheet, from Stanford University