Tumbleweed Hideout: America
Mission requirement: New Friends, Old Problems
Objective: Help the Sheriff kill all the criminals
Reward: Winchester Repeater
Tesoro Azul Hideout: Mexico
Mission requirement: The Demon Drink
Objective: Kill the Criminals and save the deputy/sheriff
Reward: Double Action Revolver
These are initiated when you come across the area while bandits are there simply kill everyone that's trying to kill you and save whoever you are supposed to.
Here is a map if you didn't buy the game. if you did then the areas marked with "danger" are the hideouts.
The link posted above came from xbox360achievements.org
Note if you finish all the hideouts in the US within 24 hours game time you will unlock the Marshal's outfit.