BioWare has released an extensive Q&A on its official forums for fans, to quell questions about Mass Effect 3's multiplayer.
The four-player multiplayer component which will "have a direct impact on the outcome of the single player campaign" as well as discuss Galaxy at War in greater detail - however, the company is keen to stress that this is entirely optional "it is still possible to achieve the optimal, complete ending of the game in Mass Effect 3 through single-player alone."
Chris Priestly outlined the multiplayer in one epic forum post, explaining how players choose "from a variety of classes and races, form an elite Special Forces squad" and how the Galaxy at War system measures 'Galactic Readiness', which can be impacted through multiplayer, and rather teasingly, "other platforms and interfaces" to be announced in the near future.
The multiplayer cooperative missions were designed by a dedicated team at BioWare Montreal, separate from the main game. You won't be able to control Shepard and co, but create a custom collection of characters from the likes of "Turians, Krogans, Asari and more... each with their own unique set of abilities."
Also confirmed is DLC for Mass Effect 3, and that your save games from the first two games won't impact the co-op missions.