Thermogenics increase the body's metabolism in order to burn fat. They do so by raising the body's temperature by about a degree. The theory is, by causing the body to create more energy, it will burn more fat. They may also have the side effect of suppressing the appetite.
The most popular thermogenic composition was the ECA stack, which consisted of ephedrine, caffeine, and aspirin. On February 6, 2004 the FDA banned ephedra for use in dietary supplements, altering the composition of most products.