Call of Duty 4: Modern Warfare exists of a whole new multiplayer part. Now players have more weapons and classes they can choose out from.
There are five standard-classes: Assault, Spec Ops, Heavy Gunner, Demolitions and Sniper. And on top of that there is a new ranking system.

The player ranks up through gaining Experience Points (XP). This XP can be earned by shooting opponents, planting or defusing bombs, launching UAV's and calling in Airstrikes. The higher the rank the more classes he can create.

A class exists of a range of weapons that can be equiped with laser vision (Red Dot Sight) or a silencer. Furthermore the player can choose a sidearm. Lastly the player can choose so called "Perks". Some perks are yet to be unlocked by ranking up.
There are a lot of Perks available, different perks have different advantages on the battlefield. For instance there are perks which reduce your sprinting noise, or perks that help you sprint longer but also perks that upgrade your weapon's strenght or speed.
There are a lot of different gamemodes in multiplayer: Team Deathmatch, Free for All, Team Tactical, Domination, Search and Destroy, HeadQuarters, Sabotage and Cage Match.