There are two possible answers to that question. There was a subgame called "kill team" that was in the 4th Ed rulebook that allowed you to create a small comando squad that was on a mission targeting something important. The opposition was a bunch of mooks. There were some unbreakable rules and some breakable ones. Breaking the breakable ones gave the mooks more cool things. They also had rules for extra gear (silencers, etc.). It's a lot like a comando scene from an action movie. The rules used to be up as a PDF over at GW's website, but they took them down when the new variant came out, IIRC.
In the new Battle Missions there is ALSO a sub-game called "kill team". It's basically a cross between a combat patrol and one of the 4th ed kill team missions. You take 200 points worth of dudes, every model is considered a sepperant unit and then you get 3 USRs to distribute as you see fit. The other side does them same thing, and you battle it out.
Read the rest of the comments to see how people feel about the second one. I'm a big fan of the first one and I don't want my bias on the second one to effect you. There are definately people who feel both ways, I'd check out what they have to say.
Her(?) name may be slightly inapropriate, but she just needs love! And I just need to eat your face!
I'm probably going to follow my usual Eldar tactic of driving around the table "circling and screaming 'I don't give a fuck' with my windows down and system up" and avoiding anything that looks scary.