Its based on...
A terrorist and counter terrorist.
You pick your side.
T's plant the bombs, CT's try to disarm it.
Or you can do kill the other team type things
or gun games, where each kill you advance in a gun.
CS:S. It's definitely not like Halo where you have about 40 shots before you ever die. Nope, this game is 2 shots and your down. It's still fun though, you just need to be really good if you're playing online and you don't want to die every 10 seconds
NOT MARIO PARTY, I remember playing that on the old Nintendo 64 consoles< that doesn't seem to be that long ago, the old cartridge games, how old school.
Nothing is great about it, 1.6 was much much better but Valve wanted to it be a main selling point on Steam so they said that all Tourneys must be held on Source.