That's a rather fuzzy topic, as the term art is in itself not so easy to define precisely.
As for what I think it is, art is any form of production that has the intention to evoke or provoke something, a sentiment, an idea, a reflexion to someone else.
For me that's how I make the difference between an artist and a craftsman. The second has the intention to produce something, maybe beautiful, maybe very skilfully, but the intention is the object in itself (take a violon, it's build to be the best violon possible, nothing more, but it's already on the verge, because the craftsman could hope his piece to be a source of inspiration for the player, more than just by the functionality of the instrument). The artist will write a piece of music not just for the sake of producing a piece of music, but when this is played and listened to, the composer has the intention to make something else pass to the listener than simply the notes. Sadness, grandeur, joy, fear, whatever.
And this can explain some very mindboggling pieces of modern art (take Duchamps). Some people do not grasp why a toilet seat is presented as a piece of art. That's art for Duchamps because he wanted to convey a message, a reflexion. That's surely not art for someone that do not grasp the message.
This sadly is often used to justify non art, or errors or errands of someone post-justifying over-intellectually something that has miserably failed (yeah, all my polaroids are rotten, but that was deliberate ;-), I have that grand scheme of showing the decay of our world. hmmm. That can be geniune, or totally fake.
And sometimes, you find cheap print-on-demand 'art photography' ready to be framed that look to me to be craftsmanship, not art.
Packshot, commercial photography, fashion top model photography, rarely I see art there. Often, superbly done, qualitative to the extreme, but conveying nothing else than an illustrative purpose.
And because Art is defined (to me at least) by what message is conveyed, everyone will be sensitive to a degree to the message, is readability, some will grasp it, some not, and some will say that's art, and other that's not art, that's documentary, and so on.
So, someone can see a photography as art, because the picture evokes something to her/him (I mean something more than just laugh or basic information), even if the photographer did not knew the picture could convey that message, and some photographers can take pictures thinking they are making art, with a purpose, a message, an intention, but no one understand or receive the intended message.
Art being a vector, the two sides have their own point of view.
My two cents ;-).
Greg