The Murderer's Job is to kill everyone without being shot by the shrieff. Your percentage to become one increases the longer you play the game.
The Sheriff's Job is to find the murderer and shoot him with your gun. If you shoot an innocent, you and the Innocent both dies.
The Innocent's job is to try to avoid getting killed by the Murderer and tell the sheriff who the murderer is. If the sheriff dies, his/her gun drops on to the location where he died and if an innocent picks it up, he becomes a 'Hero' and can shoot the murderer with the gun like a sheriff.