Team Fortress Classic -- one of the coolest mods for the game Half-Life. In it, there are two or more teams, and each (red, blue, and on maps that support it, yellow, and green) has an objective to accomplish. Each player selects one of nine "classes", which are:
1. Scout
2. Sniper
3. Soldier
4. Demoman
5. Medic
6. HWGuy
7. Pyro
8. Spy
9. Engineer
0. something called "random playerclass", kind of a misnomer because it cycles through playerclasses (1 to 9) after each death.

The game itself is hard to describe, but it is awesome with a bit of a cartoon-violence look. There is no other way to explain it unless you go out and play it!