Basic TF2 Knowledge:
It's a large scale FPS
Multiplayer only, but there's one (short) singleplayer mode
Comes by itself for around $20 on Steam, or around $30 on the Orange Box (which includes four other games. Yeah.)
There are several different game modes: Area Control, Capture the Flag, Payload, and tons of community-made modes.
There are around thirty different maps, not counting community maps.
Nine (very balanced) classes: Scout, Soldier, Pyro, Engineer, Heavy, Demoman, Medic, Sniper, and Spy.
It uses health-kit health, not recharghible health.
Around 50+ added weapons, and hats.
Despite above fact, the game is very well balanced.
EVERYTHING I mentioned is free (well, on the PC at least).
Best tip: Play as a Medic: Self healing, not much combat needed, basically just follow experienced players and learn the ropes. But please, heal the guys with the big guns.