This is a really cool bit of software.
I used this with the tortise frontend and its fantastic.
It kicks the crap put of sourcesage.
Essentially it allows you to track, monitor, back up and roll back any changes you have made to your websites source code.
Its also really smart, it can handle source merges if more than one person is editing a file at the same time, and its scary how it knows exactly how to merge changes together.
If your working for any large scale company this is a prerequisite.
http://subversion.tigris.org/