Multiplayer in GTA IV

Multiplayer in GTA IV is a new, fully fleshed-out feature for Grand Theft Auto IV. The multiplayer features:
 * A total of 15 game modes divided into three main categories (Player Match, Ranked and Party Mode), each with unique objectives and gameplay types.
 * A maximum number of players is 16, although various modes have limitations on the number of players that can join.
 * Configuration of each mode can be adjusted by the host. Vehicles types, volume of traffic, number of pedestrians, police presence, weapons, auto-aim and weather can all be controlled.
 * Characters are completely customisable. You can essentially create your own unique online character by adjusting their hair, sex, race, clothing etc. *Additional options can be unlocked by increasing your rank in Ranked multiplayer matches. However, players won't be able to customize shoes, facial hairs, even hairstyles.
 * Games are, in essence, random. Upon choosing a game mode to play, you will either be thrown into an already made game or you will start your own.
 * Killing another player will get you $100. A dead player will usually drop approximately $15. Killing yourself will cause you to lose $100.
 * Ranked matches, like everything else in multiplayer GTA, requires money to get ahead. The amount needed to gain a rank increases exponentially as you progress.

Starting Multiplayer
ACCESSING MULTIPLAYER To jump into a multiplayer match, you need to have the cellphone. (The cell, however, is given to you very early in the storyline, so this shouldn't be a problem.) To jump into multiplayer, do the following:


 * 1) Bring up your mobile phone.
 * 2) Access the cellphone menu.
 * 3) Scroll down to Multiplayer.
 * 4) Enter the Multiplayer submenu, at which point you can access multiplayer matches, customize your character or view a multiplayer tutorial.

The multiplayer tutorial will give you all of the information you need to know to jump into multiplayer games, and is a great way to introduce yourself to the dynamics of multiplayer Grand Theft Auto.

Player Match
Player Match lets you go up against other GTA gamers in every possible multiplayer mode. This is about fun moreso than bragging rights; who scores what for each game is irrelevant once the time is up. This is a great way to relax, have fun, chat with other people and enjoy Grand Theft Auto for the sake of enjoying Grand Theft Auto. It's also a good way to get a feel for the different modes before jumping into Ranked matches.

Ranked Match
Ranked Matches -- either on PSN or Xbox Live -- are for those of us who need true motivation, and a massive e-peen to boot. Every Ranked match you enter puts you up against opponents who are playing to win. The word "fun" doesn't mean a damn thing here. As you progress in Ranked matches and earn more money, your rank will gradually increase (maximum ranked level is 10), unlocking additional goodies (clothing, basically) for your multiplayer character.

Party Mode
A VIP, invite-only variant of Free Mode. Party Mode throws everyone on the same team and sets everyone to respawn in the exact same location (complete with every single weapon imaginable, as well as vehicles, First Aid kits and body armor). There are also no police in Party Mode. As the name would suggest, Party Mode is great for running around in Liberty City and causing all hell to break loose.

Source
GTA IV Net - Multiplayer: Gametypes, Modes, Online