http://www.youtube.com/watch?v=XRJLjgPExGQThe game is 2 sided. When you're on "defense" you are playing an FPS. You are a Mercenary with lethal weapons. You have a gun, you have motion tracking, you have EMF visions, you have mines, you have grenades, you have a gas mask and you can carry a backpack to refill your gadgets with. There are also other gadgets you can choose before each round, but you can only have 4 at a time.
http://www.youtube.com/watch?v=KFrMgzb4i_8This is the other side. The Spy side. My favorite side. You play offense on this side. You have no lethal weapons besides your hands. If you can sneak up on a Merc, you can grab them and break their neck. You can also jump on them, at your own risk, to knock them out. You also get gadgets on this side. You can only carry 4 on this side also. There's neat gadgets like the Heart Beat Detector, which can show you where your opponent is on the map if they're close enough. There's the Sticky Cam which is a mini camera used for recon that can also deploy a gas that knocks the merc out for 8 seconds or so. There are the grenades that you can equip for quick use: Smoke, Chaff, Flash. These are assigned to your left or right trigger, with each trigger dropping a different grenade. I carry Chaff and Smoke. You drop these at your feet like a ninja or you can also shoot them out of your non lethal weapon. The spy is faster, more agile and able to jump higher and climb nearly anything. Pipes, window seals, fence, whatever. The spies have Night Vision and Thermal Vision.
This is a 2v2 game. It was amazing when it was in it's prime and gave Halo 2 a run for it's money as far as the number of online players go.
BTW, the people playing in the vid are not very good. They're just an example to show the game play.