Simple. 1 player randomly placed on the map versus 1 other randomly placed player!
2 teams of 2, your team mate will always be in the same corner as you.
2 teams of 3, your team mate will always be in the same corner as you. Not sure about the 3rd, haven't tested much but it looks like you will be playing top vs bottom style.
2 teams of 4. Your team will be on the same side as you. Either top, or bottom.
Free for all
8 players all against each other. The most carnage you can possibly get!
Co-op Easiest, Easy, Medium, Hard, Harder, Very Hard, Elite, Insane
4v4, 4 computer players on one team locked in their settings. 4 open slots on the human side. Difficulty ranges from Easiest to Insane.
Co-op Near Imposs
2v6, 6 computer players on the hardest difficulty the game can offer with 2 human slots. Good luck!
1v7, 7 computer players on hardest difficulty the game can offer with only you to battle them. Good luck! Post a replay if you manage to be able to defeat them. Highly unlikely though. lol