In it, you'll be able to create and develop a unique character and explore the Mass Effect universe with friends. Characters won't be limited to humans either, so if you feel the need to roll-out as a Turian, Krogan or Asari you can do just that.
The entirely optional co-op missions take place outside of the single-player game, and aren't affected by your Mass Effect 1 or 2 saves. BioWare insist that they haven't compromised the solo experience, but participating in the multiplayer may make the single player campaign a bit easier, offering "an alternative method" of achieving success in the main story - the game's endings won't be changed however.
BioWare's Chris Priestly says "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.
"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."