If you look up Barbecue sauce in wikipedia, you will find that it adds flavor to meats such as pork, beef ribs and chicken as a marinade, basting or topping while being prepared for, during the process of, or after barbecue style cooking.
A great article to enjoy and learn much from: Origins Of The BBQ Sauce And Things You Should Know
Most any sauce can also be used as a standalone condiment for other foods as well.
In the United States there are several barbecue sauce variations based upon region:
Alabama White Sauce
primarily used on chicken and pork
composed of mayonnaise, vinegar, sugar, salt and black pepper
East Carolina Sauce
|thin and sharp to penetrate meat and cut fats in the mouth little or no sugar
|thick, reddish-brown, tomato or kechup-based with sugars, vinegar and spices.
thick and sweet
does not penetrate meat as much as sit upon the surface
most common and popular sauce in the US
|similar to Kansas City style but uses more vinegar and uses molasses as a sweetner.|
|a lot like East Carolina Sauce (above)
South Carolina Mustard Sauce
|primarily made of yellow mustard, vinegar, sugar and spices.|
|heavily seasoned with cumin, chili peppars, chili poser or ancho powder
lots of black pepper
touch of tomato
little or no sugar
often contains meat drippings and smoke flavor
medium thick and resembles thin tomato soup
penetrates meat easily rather than sitting on surface