Aliens are the new zombies.
Call of Duty 5 famously included the not-so-secret secret "Nazi zombies" mode, which, considering how tired the zombie genre is, was surprisingly well done. The trend continued with Black Ops and Black Ops II, but it now looks like even CoD has grown weary of the shambling undead. The "extinction" gamemode that everyone assumed would be Call of Duty: Ghosts' zombies mode, has actually turned out to be an alien hunting mode. Imgur user smtkz has leaked a series of screenshots showing us what we can expect of the new mode, which is reportedly unlocked by completing the first five missions of the campaign.
The aliens have a very "Zerg" feel about them, and from the looks of things, you'll play co-op and fight of waves of aliens that gradually increase in difficulty, much akin to the old survival mode of the original Aliens vs. Predator, horde mode in Gears of War or, if you will, that scene in Starship Troopers where they hold out against like a million bugs.
Players will pick between four classes: tank, weapon specialist, medic or engineer, and will have special abilities (such as medkits and portable turrets) that they can level up with "skill points" and cash collected while progressing through the game. From the screenshots, the object seems to be to destroy the enemy hive before being overrun.
This should certainly come as a nice little surprise to long-time fans of the series who were expecting yet another zombie mode, or Infinity Ward's signature "spec ops" mode. I've always been a fan of "horde mode" co-op, and extinction certainly looks like it would be fun to grab a bunch of buddies and see how long we can last.