Form Drag, also known as Pressure Drag or Profile Drag, is the drag caused by the separation of the boundary layer from a surface and the wake created by that separation. It is primarily dependent upon the shape of the object.
In the upper figure of the diagram, the relative wind across a flat plate results in a leading edge stagnation point at the front of the plate that contains very high static pressure. The airflow attempts to maintain contact with the surface of the plate, but the streamlines are unable to follow the sharp angles which would be required to allow them to fill in behind the plate. As a result, they separate at the trailing edge of the plate leaving a low pressure wake area behind it. The pressure differential between the leading and trailing edges of the plate causes the plate to be pushed in the direction of the relative wind and retards forward motion. This is form drag.
To reduce form drag, aircraft surfaces which are exposed to the airflow of the relative wind are streamlined. The remaining figures of the adjacent diagram show how, as the streamlining is increased, the form drag is decreased.