Simple: if it's 4x6 or 5x7 and in color, it's a snapshot. if it's in color at all, it's commercial. if it's any size and square or 8x10 or larger in black and white, it's fine art!
I'm kidding of course, but using the AIPAD show as a barometer of what galleries internationally regard as collectible, fine art, it's not too far off the mark. It's true that well established 'names' are the backbone of the fine art market, but the galleries represented at AIPAD (and that's a very large number of them under one roof) definitely do represent and promote new and 'emerging' fine art photographers as well.
As to what constitutes a fine art photograph, I've no better a definition than anyone else, but I think one knows one when you see one. To be silly again, then, I guess if a photograph serves no apparent useful purpose, it must exist because the photographer believed in it for whatever reason that decision was made. Perhaps that makes it art.