Definition: Smoking is an age-old process of cooking and preserving foods, particularly meats. This ancient process sometimes referred to at hard smoking uses salt to inhibit microorganism growth and the smoke produce an acidic barrier to prevent contact contamination. Properly salted, smoked, and stored, the shelf-life of these preserved meats is literally endless though there will be a continuous degrading of quality over time. Really old beef jerky may be safe to eat, but it tastes stale and unappetizing, which is the reason it gets vacuum packed, to prevent oxidation from destroying the flavor.
This process of smoking and preserving foods was perfected long ago, the secret, the smokehouse. It is easy to salt a pork roast and put it in a smoky room and end up with a rancid hunk of meat. And yet people without electricity or germ theory could pull this off, making hams that lasted through the year without refrigeration, without getting the slightest tummy ache. Smokehouses dating back centuries had the precision of temperature and humidity control necessary to make a ham that could be left in the cupboard all summer and enjoyed for Christmas.
It is the combination of salt and smoke that caused this kind of preservation, but either one can be used individually to hold off spoilage for shorter periods of time. In the midst of the French and Indian War. George Washington, then a British soldier, recorded that his men had used the "Indian" method of barbecue to preserve meat. They lacked the salt to properly salt and store the meat, which would have given them a few weeks. The rough smoking method they used only preserved the meat for a few days.
Washington didn't refer to this method he called barbecue because he didn't know about meat smoking or because he knew what barbecue was in our modern sense. He called it this because the meat he had procured wasn't cured and smoking in a smokehouse, but instead over an open pit, the method used in the earliest days of barbecue and attributed to the indigenous peoples of the Americas.
Smoking, as in modern hot barbecue smoking, is a cooking process. Many people will bore you with explanations of the Maillard Reaction. This chemical reaction between amino acids and reducing sugars is what gives many cooked foods their brown color and produces that cooked flavor we tend to look for. The Maillard Reaction causes meats, bread, and pretty much anything organic to gain these characteristics. While an interesting science experiment, it isn't unique to barbecue.
With the advent of modern refrigeration, we no longer need to preserve meat and while smoking in the typical backyard smoker can add a little shelf life to meats, they are best stored in the refrigerator because the amount of smoke and the lack of heavy salt doesn't mean these foods are actually preserved.
Today, smoking, in terms of barbecue is a method slow roasting with the addition of smoke flavor in an enclosed type of cooking equipment. Natural, hardwood smoke adds nitrates to meats and causes a chemical reaction in meats, most noticeably in the form of a smoke ring. Smoking simply means, these days, a method for making barbecue and other foods like jerky, smoked fish, and similar items. It is important to note that many foods sold as "smoked" never actually spend time in a smoker or smokehouse but instead have smoke flavor added to them. Smoke is vital to barbecue and nothing should be called barbecue in the classic meaning of the word without natural smoke added to the cooking environment. Unfortunately, barbecue has come to mean a flavor and not a method of cooking.