Infinity Ward has released a pretty detailed look at what Spec Ops mode is all about in Call of Duty: Modern Warfare 3. You're basically fighting waves of enemies that get progressively harder the longer you last. So it's like the zombies mode in Black Ops, but you know, fun.
I played Spec Ops mode while at E3 and loved every second of it. It's so fast paced and fun. The AI is pretty damn smart too. You can't simply stay in one spot for too long as the enemy will adapt and figure out ways to flank you. Oh, and protip, don't try to melee the Juggernaut as it (obviously) won't really work. I was pretty embarrassed when I did that and then died.