Two links from BetterExplained:
There is also Eric S. Raymond, author of "The Cathedral and the Bazaar" and "The Art Of Unix Programming" DRAFT of "Understanding Version Control"; note that information about current trends in version control and current version control systems is at the time being (17 July 2009) around 1.5 years out of date.
Added 18-07-2009:
Each of leading version control systems has user's guide kind documentation which explains basic version control concepts: Version Control with Subversion (aka "svnbook") for Subversion, Mercurial: The Definitive Guide (aka "hgbook") for Mercurial, Git User's Manual or The Git Community Book for Git. I suppose that similar document exists for Bazaar.
I would recommend also reading The Git Parable by Tom Preston-Werner (one of GitHub founders), which describes how distributed version control system such as Git could have been created from first principles (it is fairly close to actual history of Git development, but misses in few points). It helps to understand the generic concepts behind distributed version control system, and the concepts behind design of Git.