Polymer clay is essentially PVC, polyvinyl chloride. It is plastic, but until it is cured, it is a very malleable plastic. It can be shaped and reshaped a multitude of times without deterioration. As long as it is stored away from excessive heat and direct sunlight, polymer clay can sit unused for years and still be workable (though it may require softening additives if left for very long).
Unlike earthen clay, it doesn't have to be fired in a kiln, nor will it dry out at room temperature, like other clays will do. Polymer clay cures at significantly lower temperatures than earthen clays, so it can be easily hardened in a home oven or toaster oven. (In fact, the high temperatures in a kiln would ruin polymer clay and release potentially dangerous fumes.) Though it varies slightly by brand, most polymer clay must be cured at 265°F to 275°F (129°C to 135°C) for at least 15 minutes per ¼ inch (6mm) of thickness.