In science terms, a geological fault is a fracture or fissure in the Earth's crust in which there is movement on either side of the fracture. A fault line can be the boundary between two tectonic plates. There can be active and inactive fault lines.
Similarly, the type of movement along a fault line is dependent on the specific kind of fault line. Three different types of faults are normal, strike-slip and reverse faults. Normal faults are associated with rocking pulling away, while reverse faults tend to compress together. A strike-slip or transform fault has a rock block movement that is horizontal. However, faults also can be combinations of these types. Fault lines are associated with earthquakes; for example, the San Andreas Fault is a strike-slip fault.