Art. If it's just some punk kids trying to be smart and putting something dumb, then vandalism. But even names, tags, if they mean something (which they usually do to the artist) constitute art in my book.
Also alot of artists don't use private business, homes, cars, churches, and things like that as a canvas. We consider it rude. Only big places. Department and chain stores, big commercial trucks, things like that. These people can afford to paint over something they don't want there. We're sending a message, not purposefully trying to destroy things.
If the people doing it are doing it for a reason and not just drawing giant cocks and shit sure its art. Quebec has a graffiti art park that is fucking beautiful.