Counter-Strike: Source is the "revamped" version of the original Counter-Strike. At to be honest, it's better. Now equiped with more realistic graphics and physics, the game is actually mindblowing. It's make you feel like you are there more than the other CS did. Here is just a short overview.
Graphics: Obviously the biggest difference in this version over the original. Way better. Period. Actually, one of the, if not the, best graphics in any game (along with Half Life 2 of course).
Gameplay: Same old Terrorists versus Counter Terrorists. Still fun. One of the best multiplayer games around, still.
Controls: Still the unbeatable keyboard/mouse setup from Half Life that is flawless. No troubles or changes here.
Replay: It's a pure multiplayer game. What do you think? You'll be playing it for hours to come.
Overall: As an add-on multiplayer experience to an already perfect game, definately well worth your time. Not just some cheap, thrown together multiplayer game like lots of games have now-a-days. Buy Half-Life 2 and play this mod.