A crack in the earth's crust is called a fault. There are different types of faults that appear in different contexts.
Some faults cannot be seen by someone standing on the Earth's surface. That is because these faults occur underneath surface, but many faults do extend up to the surface, forming different kinds of cracks in the Earth's outer crust.
Faults are also referred to as fractures. When there is immense pressure underground, the surface of the Earth can be easily brittle enough to split apart in various ways. Sometimes, this phenomena can be of such a magnitude as to create mountains and valleys. Other times, the movement caused by the fracture is of a horizontal nature instead of vertical, so giant cracks will form in the Earth's outer layers.
Stress builds between two crustal plates when forces push them against each other. In this way, the creation of faults, or fractures, can occur over a very long period of time, or can just happen in a mere instant. When the energy is finally released from the overwhelming pressure, the ground will commonly vibrate. This is where earthquakes come from. Crustal movements and releases of energy, or earthquakes, appear everywhere on the Earth. Earthquakes are extremely common as they occur on a daily basis globally. Many of them are too faint to be felt, but some of them can be extremely severe.