Other contributors have said "How a cast is related to a mold?" is the same question as "How is a cast related to a mold?" If you believe that these are not asking the same thing and should be answered differently, click here
A mold is made in the reverse of what the item is going to be made. It forms the outside of the object is and the cast is injected or poured into the mold. An ice tray is a good example. The tray is the mold, the ice is the cast.