What "Is" Barbeque or "BBQ"?
You question has a short answer to it.

Barbeque - What you do on a gas or charcoal grill

BBQ - shredded, pulled or sliced pork, lamb or beef seasoned with a hot pepper and vinegar , mustard based, tomato based or mayo based sauce.

For more on BBQ the BBQ song - http://www.youtube.com/watch?v=6ubTQfr_tyY