What is the difference between a cast fossil and a mold fossil?
Think of it like a Jello mold. The plastic container is the is the mold, and you cast the mold onto the Jello. So lets say a couple of hundred thousand years ago you stepped in some mud and it dries. The footprint you left in the mud is now the mold fossil and what fills it in is the cast fossil. This process holds true for trolobites, coral reefs, shells, plants, etc. Here is a small animation I just found on a shell if you are still having trouble picturing it. I hope that helps.
