Mass Effect 3 is the first game in the franchise to offer a multiplayer segment in addition to the single-player campaign. 

Instead of Commander Shepard, you take the role of an ordinary soldier fighting in the war. You use the same classes as in the single-player, but each class can be one of four different races, each of whom have slightly different abilities. 

You and up to three other players fight through eleven waves of enemies - Reapers, Geth or Cerberus troopers - on one of several Firebases, abandoned Alliance bases which, for one reason or another, are important for the war effort.

During each mission, you gain experience, which is used to upgrade your powers, and credits, which are used to purchase packs. 

The level cap is 20. At this point, you can promote your character to reset its level and powers, but increase your N7 Rating and single-player Military Strength.