Multiplayer is the newest addition to the GTA series in GTA IV. In multiplayer, you can choose to do just about anything you want to do. You can play from a variety of Modes, or you can just wander Liberty City with friends in Free Mode.
In GTA IV, there are 3 main types of ways you can play games, Player Match, Ranked, and Party Mode.
In a Player Match, you can play any of the game types available with anyone for fun. These games are games hosted by players for casual play, and in these games you will not be rewarded cash that will go towards your rank.
In a Ranked Match, you can play any game types available for cash that will go towards your rank. In these matches, all money earned will count towards your over all amount. These can be played alone or with friends.
Party Mode is basically just a lobby for your friends to join before heading into Ranked or Player matches, or before starting your own. In Party Mode, the whole city is available for you to mess around in.
Rank in Grand Theft Auto IV is what shows how experienced you are in GTA IV's online mode. Your rank can only be leveled up in online ranked matches, and has become a higher rank, the more you unlock to customize your player model.
What determines what rank you are is the amount of cash you have. Every kill you get in an online ranked match will award you with cash. Cash can be obtained other ways, such as collecting it from fallen civilians, enemies, team mates, and by completing objectives.
In GTA IV, there are no maps to play multiplayer on. Instead, all game types take place in Liberty City, but having restrictions on where you can spawn in certain areas.