A fault is an error caused by ignorance, bad judgment or inattention. If you're a passenger, it might be your fault that your friend missed the exit, if you were supposed to be watching for it, not sleeping.
Fault can mean "blame" — as a noun or verb. If you say, "It's my fault," you accept the blame. Well, they can't fault you for telling the truth, at least. A fault can be a shortcoming — everyone has faults because no one is perfect — or a crack in the earth's crust, like the San Andreas Fault. In sports like tennis, a fault is an illegal hit of the ball, usually when it lands outside the playing area.