Reason it's called Source is because it's on the Source graphics engine, which was created for use in Half-Life 2. Graphics are one upgrade, but physics are now a huge thing in CS:S and CS 1.6. Behavior of movement of players, objects, and other things are now way different, as well as accuracy of weapons. As far as I can remember, everything you can buy is the same, with the exception of the pansy shield which didn't make it into this version.