Metal Clay contains binders and metal particles. Some clays come moist, pre-packaged, while others come in powder to be mixed with water. Base metal clays are fired in a kiln, in charcoal. Fine silver clay can be fired with a butane torch, but there are exceptions. Check the web for classes.
Metal Clay Gallery:
Polymer clay is not a true clay. It has a plastic base similar to PVC. Within the past few years, polymer clay has taken on a life of it's own, even though it was discovered in Germany during WWII. The Racine Art Museum (RAM) has hosted a polymer exhibit and The Pratt Institute will be offering classes this fall, taught by Donna Greenberg. The world is full of incredible polymer clay artists. It is an inexpensive craft that can be formed into masterpieces. Check out Pintrest and Google.