"Red Dead Redemption gives the player a large, open world environment allowing the player to move freely. Players on foot can walk, talk, shoot, kill, ride, run, interact with the environment, and combat enemies with firearms. Different breeds of horses are the main forms of transportation. Swimming is not an option, as the protagonist John Marston cannot swim and will drown if the player ventures too far into a body of water (though he is able to wade where the water is shallow enough).
The open world environment allows players to choose how they want to play. Storyline missions are for progress. Along with the main storyline, players can take part in random events they encounter as they explore the West. These include public hangings, ambushes, pleas for assistance, and dangerous animal attacks. Players can also take part in optional side activities ranging from standoffs, gambling, and hunting fauna for pelts and meat.
Red Dead Redemption makes use of a morality system where players have the ability to gain positive or negative 'honor'. Some of the ways to gain positive honor include taking an outlaw alive instead of killing him, or saving a kidnapped innocent. Negative honor can be attained for committing crimes including killing and/or robbing civilians. This works along with another system, 'fame,' showing and governing how people react based on Marston's honor status."
Source: Wikipedia, "Red_Dead_Redemption," available under the CC-BY-SA License.