THE HEAT...Coals...low and slow
You can use hardwood, charcoal briquettes, or lump charcoal. All three burn down to hot coals, which cooks the meat. The advantage of using wood over briquettes or lump charcoal is the flavor imparted by the smoke. Briquettes and lump charcoal are processed products and, although they do smoke while burning, have lost virtually all of the means to produce the flavorizing effect of burning wood. If using briquettes or lump charcoal, wood chips or chunks are required to produce the smoke that flavors the meat.
If you burn wood in a fireplace you know the more dense the wood is, the longer the wood burns. Most any wood will work for a fireplace. Not so with your smoker. Hardwoods, either from a nut or fruit tree are most commonly used due to the flavor imparted by the wood. All hardwoods are dense, just some more dense than others. Although a few pitmasters (you can count them on one hand) will use green wood, it is best to use wood that has aged or cured out. Most wood is best used in the range of 6 months to 1 year. If you have wood that has been around a while and is looking old and getting light in weight, it can be used as fire-starter wood. Use it to start your fire. Use good, heavy, dense pieces of wood for getting your primary bed of hot coals.
The BBQ SHACK typically smokes with oak, cherry, apple, hickory, pecan. Various herbs or onions thrown on the fire can also impart a pleasant flavor to meat. If you have a question if a certain type of wood is good to burn for BBQ, light a piece on fire and smell the smoke. If it smells good, it will probably taste good as well. Beware of hickory, mesquite, and pecan. These woods, used for an extended period of time, can and will be over-bearing. Use them sparingly!
Many smaller smokers cannot function reliably on an all-wood fire. A couple examples would be the Brinkmann and Weber Smoky Mtn. These require a briquette or lump base (usually briquette) supplemented with chips or chunks for smoke. Other larger smokers as the BYC (back yard cooker by Klose) or trailer models with larger fireboxes can operate on all-wood fires.
For those using charcoal, you will find briquettes have a longer burning time and burn cooler than lump charcoal does. Briquettes are uniform in size and you will tend to get less temperature spikes with briquettes once your target temperature is attained. Many people believe the fillers which are included in briquettes impart undesirable flavors to the meat. These fillers may include powdered charcoal, anthracite coal, clay, limestone, starch, sawdust, and sodium nitrate. Lump charcoal is basically pure carbonized wood made from wood scraps in an environment depleted of oxygen. If you take a chunk of wood, wrap it in foil, and put it in a fire, the result will be that of lump charcoal. Lump charcoal burns faster and hotter than briquettes. You will have to experiment with your own smoker to decide which form of charcoal (or wood) is right for you.
Once you get the fire going, shoot for the "right fire". Make sure to cook with a small fire which is getting plenty of oxygen. A small fire is easier to maintain and will enable you to keep your draft intake vent almost wide open. Keep the exhaust smoke stack full open. Too large a fire burns too hot and requires you to shut down the intake vent next to nothing, which will cause the fire to create resins and creosote. Resins and creosote are present in a "dark" fire. The "right fire" will actually give off very little smoke. If it is smoking, make sure it is white or blueish-white smoke and not dark.
You can also use a gas grill...shsssh...it is true! Keep the flame low and if you have two sets of burners, use only one. You can add well soaked wood chips to the burners as you cook and make sure you keep the ribs off to the side or on the side with the burner that is off...INDIRECT LOW HEAT is key with gas grills. Keep the cover on while slow cooking. You can always finish off your ribs over medium flame while basting with sauce until you get a nice carmalized coating.