Statements like "he can eat a lot because he has a fast metabolism" are somewhat misleading. Metabolism isn't about the digestion of food in your stomach; it's the name given to all of the chemical changes that occur in an organism's tissue cells. And there are basically two kinds of chemical changes that can occur: Tissue matter can be built or it can be broken down.
Anabolism
These processes are called anabolism and catabolism. More specifically, anabolism is responsible for an organism's growth, its maintenance, and the repair of its tissues. So when you cut yourself and your skin scabs over and heals, you're seeing anabolism at work. Or if you grow taller or wider, that's also evidence of anabolism.
Catabolism
In order to build tissue and repair it, however, anabolism requires energy, as well as material with which to build. This energy and building material comes from the breaking down of larger, more complex material, a process called catabolism. As well as producing energy and building material, catabolism also breaks down material for excretion from the body.
Both anabolism and catabolism are necessary for survival and good health, and understanding these processes can help one make good decisions about exercise and diet. For example, when an athlete trains her muscles her muscle fibers break down. However, the daily recommended amount of protein is all that is needed to meet the anabolic and catabolic needs of the body and thus repair muscle fibers.