Grilling requires a fuel that can achieve extremely hot temperatures for optimal results. This can come from charcoal, gas, or even a campfire. The characteristics of the fuel, and the flavors they provide, can change your grilling experience dramatically.
Barbecue, on the other hand, is all about low and slow cooking where the fuel is a major contributor to the end flavor profile. In BBQ there are usually two fuels used, a starter fuel for the heat and then a second fuel for smoke and flavorings. The second fuel is most often wood with Oak, Hickory, Mesquite, Pecan, Fruit Woods, and Maple being the most common varieties. Regionally you see one wood favored over the other, in the Pacific Northwest Alder wood is king, while in the Midwest Hickory takes center stage, and in Texas Mesquite gives their Que its distinct flavor.