Information for CVS Users
CVS and Mercurial differ most significantly in that CVS imposes a centralised model of collaboration, whereas Mercurial offers a decentralised model. With CVS everyone checks out from the same repository, works against that repository, commits to that repository, and the repository must always be available for all but the most elementary version control operations. In contrast, with Mercurial, although everyone can collaborate via some central repository, their checkouts - created when they start to work on that repository's content - are actually separate repositories in their own right, and they are free to clone them to make new, distinct repositories. Moreover, unless explicitly requested, all version control operations operate independently on the local checkout: users don't require other repositories (such as any original upstream or central repository) to be available; they have everything they need to maintain a versioned copy of their work.
Here's a list of documents that may be useful to those familiar with CVS:
CvsConcepts - translations of CVS concepts into Mercurial terms
CvsCommands - equivalents of commands provided by CVS
CvsLikePractice - to continue working a similar way you always did, but benefiting from Mercurial's advantages
Glossary of terms (useful if you're not used to distributed source management)