When we think of footprints we usually think of the impressions left when an animal walks upon an appropriate surface.  These indentations in the ground are called natural molds or are referred to as being in positive relief.  Another type of footprint preservation occurs when sediment fills in a natural mold or positive print, eventually creating a natural cast (negative relief) which is essentially a mirror image of the original print (natural mold).

Figure modified from figure 3.1 in Lockley (1991) Tracking Dinosaurs

Sometimes the weight of the track-producing animal affects the sediment layers underlying the immediate track surface and can produce what are known as underprints or ghost prints.  Underprints are often less well-defined than original prints and lack features such as skin and claw impressions that can be present on the original footprint.  When working with fossil footprints it is essential to be sure what type of footprint preservation you are dealing with.