Final answer:
Faults are brittle fractures in rocks across which relative movement displaces the crust on either side, occurring due to the movement of tectonic plates. They can be divergent, convergent, or transform, causing earthquakes and contributing to mountain building.
Step-by-step explanation:
Faults are brittle fractures in rocks across which relative movement displaces the crust on either side. A fault is a single brittle fracture, whereas a fault zone has multiple brittle fractures. They occur due to the movement of tectonic plates, which can be divergent, convergent, or transform. In divergent boundaries, plates move away from each other; in convergent boundaries, plates move toward each other; and in transform boundaries, plates slide past each other. The motion along fault zones can cause earthquakes and contributes to mountain building.