Spec Ops was first introduced in Modern Warfare 2, allowing players to run through maps and chase down goals. Spec Ops changes things a bit, making it a survival mode in the vein of Gears' Horde. Set in all sixteen of Modern Warfare 3's multiplayer maps, you and a buddy tough it out through waves of enemies with one intent - survive.
Starting with a group of low-level soldiers, each wave of Spec Ops mode gets increasingly difficult. Each passing wave is announced over the radio, offering a broad idea of what to expect. Then you see the red dots begin to pop up around the mini-map. Soldiers give way to suicide bombers, as well as packs of dogs, attack helicopters and even a pair of heavily armored Juggernauts.
If a player falls in battle, a "Bleed Out" timer is activated. If your teammate isn't able to revive you in time, the match ends. Based on both of my play-throughs (I managed to hit wave 9 in one, 24 in another), I found that death usually came really quick. There's more than enough time to charge across the map and revive a teammate, though in later waves enemies tend to bunch up, slaying anyone stupid enough to come close to the pack.