From the campaign:
Well, it's about time you showed up there, lawman. We have a whole mess of villainous varmints just waiting to be brought to justice!
This here game is Posse: Wild West Justice, a card and dice game where you assemble your posse, get your assignments and then get to bringing them low down lawbreakers in so you can collect the reward money.
You'll be competing with other lawman, just like yourself, and whoever earns the most reward money, that there's the winner.
Your posse can be made up of anybody from deputized town folk, to rugged cowboys. If you're lucky, you might even get some of them super gunslingers who will bring special abilities that will make your posse even stronger.
Be warned, though. Some of them outlaws can be meaner than a rattlesnake and twice as deadly. You'll need all the luck and skill you can muster to bring them some Wild West Justice!