You can see the fault responsible for the earthquake in Haiti as a short diagonal line in the upper left part of this radar topography image. The city of Port-au-Prince is located just below and to the left of this line. See this feature from JPL for a more detailed explanation.
Large faults such as this are the points of contact between two massive plates of the Earth's crust. When the plates move, the result is often an earthquake. See Plate Tectonics, the Cause of Earthquakes to learn more.
