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.)
Basically, the weapon specialist deals 20 percent more bullet damage; tanks absorb damage, engineers secure the drill and medics revive everyone. "The most successful teams will consist of a variety of classes, using different abilities to best complement each other and adequately defend against the alien horde," Infinity Ward says.
The game, of course, arrives on Tuesday.
Call of Duty: Ghosts Intel—Extinction Class Overview [Call of Duty.com]