When it was first announced that Mass Effect 3 was going to have multiplayer, many gamers including myself were concerned that BioWare was going to include some form of deathmatch that felt tacked on and half-hearted in comparison to the rest of the game. Thankfully, Mass Effect 3 dodges that bullet and instead provides an incredibly well-put together cooperative mulitplayer mode.
ME3's multiplayer sends you to the front line in the intergalactic war against the Reapers, but doesn't really have much of a story other than setting you up as a member of a special forces unit completing tasks on the side to help the war effort.
The particularly interesting aspect of the multiplayer is how it affects the single player campaign and your Shepard's ability to take on the Reapers. There's a galactic map that gives a percentage value of each sector's status and how they're holding up in the Reaper War. Successfully survive a few missions, and the "readiness" rating across the board goes up, giving a bonus to whatever war assets you've collected so far in the single player mission, meaning when you reach the later parts of the single player game, your effort to retake back Earth and defeat the Reapers will have that much higher level of success. However, while it's certainly an interesting idea, the implication that in order to try and get the best results in ME3's main story, you have to dedicate time to playing an aspect of the game that may not normally appeal to you, doesn't entirely sit well, and feels like it should've been an optional mode to include rather than a requirement.
The style of the cooperative missions is similar to Gears of War's "Horde Mode", with ME3 pitting you against 11 waves of enemies that become progressively harder as the mission goes on. For the most part, the multiplayer gameplay plays out pretty much the same as the single player, except there are other people (hopefully) watching your back instead of squad mates controlled by the AI.
To start, you'll have access to all six of ME3's classes, deck out your character with weapons and choose your abilities, and then jump right into the fray. Combat's an intense experience, as you'll start off fighting against cannon-fodder enemies only to be battling against much more aggressive and harder-to-kill foes a few minutes later, like armored Reaper Brutes and turret-dropping Cerberus Engineers.