Graffiti and Art

Someone spray painted an alien on a side of the overpass near our house. It's the look on the alien's face that gets me. I cannot get on route 70 without seeing it and it always makes me smile. There's just something about it. Is it art? Is all graffiti art? Is some? Is it the intention of the graffiti sprayer? Is it the result? Is it the relation to the viewer? Does the fact that it is graffiti make a difference?