The concept of smoking food has been around for ages, as man learned that smoke preserved and cured their freshly-hunted meat. They also discovered an added bonus: it makes things taste awesome, and so smoking has evolved into a true alternative to other barbecue styles.
While there are various types of smoking, the concept remains the same. Wood, usually oak or hickory, is burned so that the heat and smoke are exhausted into a separate chamber where food is stored. The indirect heat allows the smoke to impart worlds of good flavors. However, food takes longer to cook with this method since the heat must travel farther. Nevertheless, if you've got the time, smoking is a worthwhile way to get your barbecue on, and will likely score you some points in the process.