Mike Zornek, fellow Mac geek and Philadelphian, has
created a very good
screencast that covers installing, and using, Subversion on OS X.
A versioning system that allows a large group of people to work on the same files without worrying about overwriting each other's work. It is very useful in development shops were more than one set of hands touches code.
Take a look at the screencast (it is about 15 minutes long) and get a good intro into both Subversion and versioning in general.