The Meal or the Cooking style
To start with, BBQ is the odd acronym for Barbeque or the proper term Barbecue.
Depending on if you are describing an event such as a backyard barbecue or if you are talking about the cooking styles of barbecue, there are still many nuances.
There is a difference between grilling and smoking
If you are going to an event in the south and calling it a barbecue, there is a higher expectation than just throwing some burgers on the grill. This is intended to be a longer event and almost certainly includes meat that has been smoked for a long time.
Barbecue by most standards utilizes parts of the animal that tend to be tougher and because of this, requires a longer cook time at a reduced temperature. Since it will be cooked for such an extended period, it provides the opportunity to infuse a smoky flavor into the food that can also take hours. This also adds flavor to some meats that would otherwise be bland if cooked without.
Since barbecue enthusiasts love the smoky flavor, they will tend to smoke food that ordinarily would be just fine on the grill or in an oven. Chicken, steaks and don’t get me started on Thanksgiving turkey. Even side dishes such as Mac & Cheese or fair game.
Time is the biggest difference. In the South, most people refer to BBQ as the food and event. So when you invite them over, they not only have an expectation that the food has been cooking for hours, but that they will likely be enjoying a cookout that lasts the afternoon and evening as well.
Four basic elements to barbecue
A meat or vegetable.
Since you want it to add flavor, certain fruit trees and other specific hardwoods are known for their unique smoke.
This an consist of a marinade, rub, sauce or a combination.
While many people do smoke cheeses and other similar cold foods, BBQ is meant to be cooked.