Molds are a simple way to make multiples of the same (or very similar) items. Highly detailed molds enable you to replicate a favorite shape with reliable results. More basic molds allow you to build your own design by stacking or otherwise combining shapes.
As polymer clay grows in popularity, so too does the variety of polymer clay tools on the market. The polymer clay section of your local craft store is probably stocked with a selection of molds made especially for use with clay. (Flexible molds are popular because of their ease of use.) You can also look for molds in the resin, soap-, candy-, and candle-making aisles of craft and hobby stores. For even more ready-made molds, type "polymer clay mold" into your favorite search engine and browse to your heart's content.
If you can't find a ready-made mold to suit your desires, or if you wish to save your money for something else, you can always make your own mold. It is possible to carve a shape to use as a mold, but an easier way is to find the shape you want (or sculpt and cure it in polymer clay) and make a mold from that.
Molds can be made from plaster, silicone, Sculpey's Super Elasticlay, and other materials. Polymer clay-particularly scraps left over from other projects-can also be used to make molds. Regular clay will yield a rigid mold. For a more flexible polymer clay mold, try Sculpey Super Flex "Bake & Bend", which can be used straight from the package or mixed (commonly in a 1:1 ratio) with another durable clay, such as Kato or Premo.