The difference between a mold fossil and a cast fossil is that mold fossil is formed when an object is placed into soft mud and is removed by decomposition or physical sources; a cast fossil happens when a mold fossil fills up with sediment. A cast fossil is three dimensional, and a mold must be present for the cast fossil to exist.
Fossils are the largest indicators of things that occurred in the past and give a glimpse of what a place looked like in ancient times. Fossils happen when the hard parts of an animal, organism or even a rock sit within mud and sediment for a long time. Nearly all fossils are made from the hard parts, such as bones and teeth, because softer parts of animals and organisms do not last for as long a period of time. The soft parts decompose quicker and will not be left in the same place for a long period of time. Trace fossils are the most common types of fossils and are not the true object; they are simply an impression of the object in the earth. Footprints, burrows and resting spots are the most common areas for trace fossils.