After a premature reveal of Mass Effect 3's multiplayer yesterday, BioWare has lifted the lid and detailed the mode which has been dubbed Galaxy At War.
This mode will be separate from the single player campaign and will allow four people to team up and take on co-operative missions. You won't be playing as Shepard either, you'll be able to make your own unique character which means you can play as a Krogan if you want. Progression will increase their "Galactic Readiness", which will determine humanity's ability to fend off the Reapers.
"Galactic Readiness" will also be affected by campaign saves from Mass Effect and Mass Effect 2 and it will be tied in with your ability to get the game's perfect ending.
Source: BioWare Social