While you were fast asleep in bed, a jittering flock of homicidal mechanical men infiltrated Team Fortress 2, signalling the beginning of the Mann vs Machine co-op update. Of course they did. The latest and final Valve post on the matter discusses some of the finer details behind the game's Mann Up mode: a semi-premium version of Mann vs Machine that rewards robot-smashing players with exclusive new hats and accessories.
There's an extensive Mann Up FAQ to be found on the site, but essentially there'll be specific collections of maps called Tours of Duty, completing a Tour of Duty will earn you a rare (and exclusive, they won't drop elsewhere) item, and accessing a Tour of Duty will require a Tour of Duty Ticket, which costs 59 pence (99 cents).
Tickets are only consumed when you receive one of these rare items, and you'll only receive an item when finishing an unfinished Tour of Duty - you're essentially purchasing your end of mission reward, and running a gauntlet to retrieve it. Even if you fail, you'll keep your Ticket to try again.
There are also Squad Surplus Vouchers for £1.29 (or $1.99), which will reward you and every other player with a rare item upon successful completion of a mission. Again, failing a mission having used a Voucher doesn't consume the Voucher, meaning Mann Up mode dodges the toxic brain-dredge of gambling and instead becomes a sort of win-win claw grabber machine, with waves of robots instead of a claw and hats instead of stuffed Scooby Doos. These new items are cosmetic, usually bits of robots crudely fashioned into fetching hats and brooches, wearable across all of Team Fortress 2 and available to trade.
Of course it would all come back to hats.
Mann vs Machine is live now, go play.