Mass Effect 3 co-op will not feature Commander Shepard
See all those awesome characters at the top? None of them will be available in the Mass Effect 3 co-op. Instead, you are tasked with creating a custom co-op character. Therefore, whether you want to be a Turian, Asari, or even Drell; it will be a unique character, different from the main storyline. Bioware also wishes to assure us that their "priority and focus with Mass Effect 3 has and always will be to deliver a complete and satisfying single player experience."
This new four player co-op mode will be a part of the new Mass Effect 3: Galaxy at War system. Not only will your "Galactic Readiness" level be affected by the choices made in the main campaign, but it can also be affected by your success in the co-op missions. The multiplayer will not be the only additional component to the Galaxy at War system. It specifically reads in the source:
I wholly expect there to be some sort of iOS game and Facebook game that will also tie into this Galaxy at War business. The idea of making a Krogan named Speedracer has peaked my interest. Keep an eye on the site, as we will be watching this very closely.