Well, van Gogh is definitely art. If you like Van Gogh is a different story, but what you like and don't like is not really a good indicator. I write books and computer games, and I consider both a form of art... That is if written with the heart and from artistic point of view and not just to fill your wallet, but hey, doesn't the same count for a painting?
There is "art" that makes me frown though. Take the picture I just showed. It's a big roundabout in the Dutch city of Tilburg. The house in the middle of the roundabout is considered "art". It's just a small empty house. All there is inside is a stairway between the top floor and the ground floor and that's it. It also moves around. Many of the locals deem it a waste of money, and which also distracts from the traffic situation, and that specific roundabout is a dangerous one. There are traffic lights there for a reason.
I've also heard that a painting called "Het Zwarte Vierkant" (The black square) was sold for millions, and all it has is a black square. I even heard of canvases full of shit (literally) were sold for millions and then I deem myself an idiot for not coming up with that myself. I'd be a f**king millionaire now if I did.
Those are of course, pretty clear examples. Look up the Dutch painter Karel Appel and tell me if you think his work is art. I guess it is, but personally I don't like it.... Daring a clear line I cannot... Art is always a matter of personal perspective after all.