The earthquake started around 4:53 pm 15 miles southwest of the Haitian capital which is Port-au-Prince. Haiti had not been hit by an earthquake since the 1800's so this is why the earthquake was pretty much unexpected. Geologists blame the cause of the earthquake on the movement of Carribean Plate Tectonics triggering a huge disturbance. The map below shows the intensity of shaking and the degree of damage caused by this Haitan earthquake.
The earthquake created 7.8 billion-8.5 billion dollars in damage. It damaged airports, roads (paved roads) and destroyed more than 100,000 homes. Schools and government buildings were also destroyed as well. Haiti was already not a very wealthy country and the fact that the earthquake struck as the country was growing again as a whole affected a lot.