Deathmatch- Free for all
Siege- One side attacks castle, other defends. Various check points, inluding a gate that can be shut and opened. The defenders have a 20 second respawn delay.
Conquest- If you have played Call of Duty 4, or any other game with "Domination" mode, this is exactly the same.
Capture the Flag- Haven't seen a server for this, but it's self explanatory. 2 flag CTF