Smoking is a low and slow process of cooking that uses smoke to add flavor and to tenderize meats. Smoking usually lasts for more than a 30 minutes a pound, but can be longer. Many good, lean cuts of meat would dry out and become inedible after being cooked for this long. However many really bad cuts of meat, full of fat and connective tissues (Collagen) come out tender, flavorful and down right delicious.
These meats are tough, chewy and generally so poor in quality that they are undesirable for cooking. Barbecue takes advantage of the high fat and connective tissues in these meats to make them great. During the long, slow cooking times of smoking the fat melts and the connective tissue breaks down. This sweetens the meat and keeps it moist during smoking. Good cuts of meat like pork tenderloin, a steak, or a good lean roast don't benefit from this type of cooking and should be cooked differently.
If you are new to smoking I suggest a small pork shoulder roast like a Boston Butt or a picnic. Ask your butcher, he'll know what you're talking about. These cuts are generally forgiving and relatively inexpensive. As you learn you can move to larger and more difficult cuts and before you know it you'll master the art of true barbecue.