During this mode your choices and actions will have a "direct impact" on Shepard, in spite of the fact that you won't actually take control of him. Instead you'll assume the role of an elite Special Forces soldier with a selection of different classes and races to choose from, including Turians, Krogans, Asari and more!
Here's a snippet from the press release:
"Join your friends in the all-out galactic war to take Earth back. The universe of Mass Effect expands with the addition of new co-op multiplayer missions, playable over Xbox Live, PSN and PC internet. Players can choose from a variety of classes and races, form an elite Special Forces squad, and combine their weapons, powers and abilities to devastating effect as they fight together to liberate key territories from enemy control. Success in multi-player will have a direct impact on the outcome of the single player campaign, giving players an alternative method of achieving ultimate victory against the greatest threat mankind – and the entire galaxy – has ever faced."
Check out the F.A.Q courtesy of BioWare forums community coordinator Chris Priestly:
Why include 4 player Co-op multiplayer in Mass Effect 3?
Being able to explore and fight alongside your friends in the Mass Effect universe has always been something we thought would be fun and compelling, and many players have asked for it for a long time as well. Mass Effect 3 is the best place for us to introduce multiplayer through co-op because of the premise of the game – all out galactic war.
Why only 4 player co-op? Why not a versus mode?
We have always maintained that we would only add multiplayer into the Mass Effect series if it made sense and did not compromise the power of the single player campaign. Fighting together against a common threat was the multiplayer experience that made the most sense for Mass Effect 3. The way we have designed co-op as a way to take control of key conflict zones in the galaxy is a natural extension of the premise of Mass Effect 3.
Why did you decide not to include it as a part of the main campaign?
Our priority and focus with Mass Effect 3 has and always will be to deliver a complete and satisfying single player experience.
Does the addition of co-op multiplayer missions impact the scope or quality of the single-player experience?
No. A dedicated team from our recently formed BioWare Montreal studio has been focused on creating the multiplayer game features while the main game continued to be developed by the team in BioWare Edmonton. Both teams are integrated under the same leadership group that produced Mass Effect 1 and 2, led by Casey Hudson. BioWare remains dedicated to delivering one of the most amazing single-player campaigns gamers have ever experienced.
How did developing multiplayer impact the single player game?
BioWare is dedicated and focused on delivering an engaging, fun, and action-packed experience for Mass Effect 3, one that lives up the BioWare standard. To reach that level of quality, last year BioWare opened a studio in Montreal that is home to designers, programmers, engineers, and other developers. Both studios work together as partners, lead by the core Mass Effect team, unified in a single vision. Under the direction of Casey Hudson and other team veterans, both studios make contributions to both the single player and multiplayer modes in Mass Effect 3. Rest assured that no compromises were made to either of these modes in the development of Mass Effect 3.
Which characters can I play in co-op multiplayer? Can I play as Commander Shepard?
Commander Shepard’s part in the war will take place in the single-player campaign, as will that of other beloved characters in the franchise such as Garrus, Ashley, and Liara…these characters do not appear in the multiplayer missions. In multiplayer, players will create custom characters to fight on different and unique fronts in the war. This will include the ability to play as favorites like Turians, Krogans, Asari and more… each with their own unique set of abilities.
What if I don’t like multiplayer – will my experience be negatively impacted?
Mass Effect 3 is a complete, standalone game that will deliver a satisfying story experience, even if you choose not to try multiplayer. The Mass Effect 3: Galaxy at War system and all of the individual components are meant to complement that amazing game and can be enjoyed on their own or as part of the Galaxy at War experience.
What if I am not good at / do not like multiplayer? Will my readiness rating go down?
ME3 is a story about a war against overwhelming force where the most you can hope for us survival. The more you do to fight that war, the more you can change that story into a more optimistic one. You can reach the highest levels of success in the single player experience alone, but Galaxy at War gives you alternative ways to get there. It’s about choice, and allowing players to find their own ways to stay immersed in the Mass Effect universe.
Will you be adding any additional maps or modes through DLC?
We can’t comment on specifics right now, but can confirm that we are planning on having DLC for Mass Effect 3.
Do save games from ME1 or ME2 impact the co-op multiplayer missions?
Do characters level up in co-op multiplayer? What is the progression system?
Character progression, weapon upgrading and levelling up is present in co-op. We’ll release more information on this topic in the months leading up to launch.
Is there more info about the other platforms of Mass Effect 3: Galaxy at War?
We are not going into details about the other components of the Galaxy at War at this time except to say we are designing each to make sense for that platform. Each component will be able to affect a player’s “Galactic Readiness” level in a different way. Again, participation in any or all ME3:GaW elements is entirely optional.
Mass effect 3 is slated for release on 09 March 2012 for Xbox 360, PS3 and PC.