Extinction, the horde mode replacing (or so it seems) zombies for Call of Duty: Ghosts will feature four classes specific to the mode, according to this post yesterday afternoon from the game's developer.
The classes are rather meat-and-potatoes: Weapon Specialist, Tank, Engineer and Medic. The first two are your offensive classes, the second two are support. Of note, the Engineer can armor the team's drill, which is the weapon used to destroy an alien hive. (You can see that in action, sorta, in the official announcement video here.)