(for those who've never played it, or don't have Ghosts yet...)
Basics: 'Creatures' (aliens) have invaded earth via "kinetic missiles". Your job is to use a laser-drill to destroy hives set up by these Creatures. Naturally, when you plant a drill, several waves of creatures attack you in attempt to destroy you AND the drill in order to protect their hives. Each area (3 of them) has what's called a Barrier Hive. The Barrier Hive requires helicopter support to bring down, and you have to fend off several waves of creatures. As you progress, the creatures get tougher. There are Creatures called 'Scorpions' that are much larger than the normal ones. They're a bit yellow and shoot toxic gas at you AND the helicopter that supports you. The poison is a liquid that if you stay in its pool, the gas continues to hurt you. The toughest Creatures are called "Rhinos". They're slower than other Creatures, but HEAVILY armored and powerful. Once you eliminate 14 hives, you eventually plant a nuke at the main hive and have to RUN ALL the way back to your original infiltration point for extraction via the helicopter. It's TOUGH, but if you have a good team that knows what they're doing, you can do it.
Your Team: Like the Multiplayer, you begin at Rank 1, and increase in rank as you gain experience by killing aliens and completing Challenges. You can do a solo run of Extinction, or you can do CO-OP. You can have up to three teammates with you to help you destroy the hives. Each player has his/her option of choosing a Class before beginning. Each Class can be upgraded during game-play by using Skill Points. Skill Points are awarded for completing in-game challenges, and for destroying hives. There's a Soldier, Tank, Engineer, and Medic. Each has its own advantages. The Solder Class essentially combines Quickdraw, Slight of Hand, and 'Stopping Power' (increased bullet damage, MW2). The Tank Class gives you more health, 25% incrementally. The Engineer makes Traps last longer, and provides an option for adding armor to the drill. The Medic Class allows you to regenerate downed players more quickly, and when fully upgraded, the poison gas from Scorpions won't continue to deplete your health.
Weapons: Each player begins with a pistol. WHICH pistol you begin with can be chosen once certain ranks have been achieved. There are a variety of weapons available at different spots around each area. Each weapons has a cost associated with it. There are searchable boxes/trash piles to find hidden weapon attachments, cash, and other items. Some include: Rapid Fire (my personal favorite), Burst Fire, Muzzle Break, Flares (used to captivate the creatures), Hyno Knife (hit an alien with it and it'll fight for you), and a Grip. Each Button on the D-Pad is assigned for a SPECIAL weapon/abilities (officially designated "Abilities") option of your choosing. As you rank up, these options get more powerful, but more expensive also. Things like Sentry Guns, Trinity Rockets, a Shield, Grenade Launchers, Ammunition Types, Armor, and Special Upgrades.
That's pretty much the jist of it.

Enjoy.