Call of Duty is a traditional first person shooter set in a real world setting. You use real guns and fight in realistic areas and locations. Unlike other modern FPS games releasing in 2007, COD does not feature a dedicated cover system, but instead uses the same crouch/prone positioning system in earlier COD games. New to the series is the introduction of a ballistics model, this means that bullet strength is analyzed and can impact the environment, even shooting through certain materials making cover a relative thing.
Following the tradition of the franchise, COD4 has you take control of multiple infantry-men fighting from different perspectives of the same war. You'll take control of an American and a British soldier as they try to take down a rogue Arab terrorist, and corrupt Russian military general. Most of the action sees you taking a role in a tight squad in battles that aren't as big as WW2 ones, but still involve dozens of enemies at any one time. The story takes place over the course of 12 different levels and is told through the use of realtime cutscenes, events unfolding during the levels, and certain details fleshed out in the briefings between missions. Level events can occur even if you're not around to see them, meaning you can miss parts of they story if you're not at the right place in the right time. After the main campaign has been completed an arcade mode unlocks that is scored based giving you multipliers and extra points for stringing kill combos together. This is similar to Halo 3's metagame.
Multiplayer has been a huge part of the Call of Duty series since the beginning, and Modern Warfare is no different. One of the big new changes with COD4 is the introduction of upgrades. As you play more games and accomplish goals such as killing certain number of enemies, you'll rise in military rank - from a lowly soldier to Captian and beyond. As you rise in rank you gain access to more abilities and "perks" as they're known in the game. You have three slots for perks, each slot with its own set of perks that are unlocked as you rise in ranks. Weapons can level up as well. The more you use a particular weapon and gain more kills, the more experience you get with the weapon which unlocks more upgrades and accessory options letting you add more ammo or lower the recoil. All of these upgrade options come together through the game's custom class mechanism where you can choose every aspect of your character's loadout including weapons, grenades and abilities. Similar to other games there are preset classes available, though the main focus is on creating your own that suits your playstyle the best.
Multiplayer is competitive only with no co-op modes available. Action takes place over 16 different maps that support between 16 and 18 players. Six different game types are available ranging from standard deathmatch to more objective based modes that are reminiscent of bombing runs and assault. And similar to Halo 3 each game type can be further customized with various options to increase longevity. One of the modes available is known as "Hardcore" in this mode you have no HUD, and bullets deliver more damage to give a more realistic feel.
Finding matches in Modern Warfare is again similar to Halo 3. It uses a lobby system where you can group up with some friends and go between matches together. You can also search for custom games that are unranked. Both the PS3 and 360 version feature the same matchmaking system, with the only difference being the 360 version uses Microsoft's TrueKill metric to aid in bringing players of similar skill level together.
Infinity Ward stresses that all three versions of the game are nearly identical. If you have a top of the line PC, then that version will look the best, but for the most part they all look the same running at a constant 60 frames per second. The PS3 version supports both USB and Bluetooth headsets, but does not support the upcoming Dualshock 3 rumble, with no plans in the future.