Has various multiplayer options:
LAN
Directip
Play by email
Pitboss
Gamespy lobby (online).
Most ppl use gamespy, you can see all hosted games in a list and join them, you can add ppl in your friends list as you come across them and track them where they are or what they are doing (ie ingame, offline, online in lobby, etc...).
You can play public games or private you can play on mods too, if you want a private game all you have to do is let the host set a password so only people you know can join.
Theres 2 mp modes: Simultaneous turns, Each player own turn, the last one is same as in single player: once player 1 is done with his turn, player 2 does his turn, once all players have done their turn the game advances one turn, and starts again at player one.
And the 1st one is that every player takes its turn in the same time, game advances when everyone is done with his/her turn.