Survival Mode in spec ops, which is playable solo or in co-op, contains endless waves of increasingly more difficult enemies. The first wave is just infantry with standard weapons, then they get better weapons and more accuracy, then its infantry and dogs, then dogs and infantry some with C4 strapped to them, then helicopters to shoot down, then juggernauts, then juggernauts with riot shields, and even more. It's constantly throwing new tactics and enemies at you.
Now the cool thing is we have progressive ranking in Spec Ops now, so you'll be able to rank up, unlock new weapons, attachments, gear (like turrets, claymores, armor), killstreaks (like calling in a squad of delta to help you defend or airstrikes) so that you can change up your tactics as you rank up.