The key here is distributed. Basically it means that every working copy is a full repository clone itself, so you can work and commit offline then push to the main repository when you see fit. See Wikipedia to read more about distributed systems.
As I was searching for a good VCS to manage local stuff, I literally tried them all. I chose Mercurial because it was distributed, fast, easy to use, open source and written in a decent language.
Once hg1 is installed, no need to set up anything to put a folder under revision. Just type
hg init or select init in the TextMate bundle2 then add and commit all the files and it’s done. All the repository metadata are contained in a .hg folder.
Recently, I’ve been using Git more than Mercurial.