Faults are divided into types based on the relative movement along the cracked rock. Most faults tend to dip into the Earth at an angle, and therefore have an upper and lower side. The upper side or roof of the fault is referred to as the hanging wall while the lower side is called the footwall. If the angle of the dip isn't 90 degrees, then the land which appears to be hanging over a lower portion before movement is called the hanging wall. Normal faults are when the hanging wall has moved down relative to the footwall.
Normal faults form when tectonic forces stretch the Earth's crust, pulling it apart.
A graben is a wedge-shaped block of rock that drops between a pair of normal faults. The normal faults around the graben are called horsts. This resembles two mountains separted by a flat valley. The mountains are the horsts while the valley in between is the graben. Normal faults and grabens are typical of divergent boundaries at mid-oceanic ridges and continental rifts where the crust is stretched.