Mold making is the process used to duplicate three dimensional models or objects. A model can be any object that has some depth to it. The original model can be made from almost anything. If you were making your own object to mold, you might use clay, rock, or plastic. Through the use of a mold making material a negative, or a reverse, of a model part is made. That negative can then be used to cast a second part that is the same as the original part in size and shape.
Simple parts such as relief sculptures can be duplicated by using a one part mold. The casting material is poured or pressed into the mold indentation. What the casting material is can range from clay, soap, chocolate, concrete, to liquid plastic, play-dough, to metal castings. You can make two part molds or even three or more part molds. Mold sizes can range from a few millimeters to many feet in size. More complicated molds may include several mold parts, rigid parts of the molds, inserts, a variety of resins and fillers, and a lot of setup and thought.
Mold making is used in industry to make duplicates of a wide variety of creations ranging from car parts to Christmas ornaments. Why you would want to make your own molds depends a lot on what you are making. At home mold making is typically performed using a rubber mold making material that can be poured on or around your object that you want to duplicate. The mold can then be filled with casting materials of your choice. For example, many cake decorators or chocolate makers use molds to create unique shapes from their chocolates or use molds to shape fondant into special shapes. Home soap and candle makers enjoy duplicating unique shapes and designs that cannot be found. Hobbyist, such as those in the train hobby, enjoy making components for their trains or improvements to the backdrops for the train sets. Or fishermen enjoy making their own unique fishing lures.