I think a piece of art becomes a work of art if it can with stand the test of time. I am sure that many of the classic works of art were just considered a piece of art to start with.
I am sure that other editors will agree too with #4 - it is all but impossible to find an agreement with what constitutes "art". This is because it is so subjective, and when you have "modern art" which really pushes the boundaries of what we consider art to be, it becomes even more confused. I think #3 makes a number of very helpful points however that clearly point towards some kind of tenuous definition. When we think of what constitutes art in its broadest term, it is important that as a general guideline it stands the test of time. I am very suspicious as a literature teacher of so many "modern classics" that are unheard of a year or two later - that in itself is a big indicator.
I think that there it is really difficult to find consensus on the definition of art. At some point, we might have to agree that the determination of artistic quality might rest with the criteria used. If one can assert a set of criteria used to determine what actually constitutes as art, perhaps this becomes the starting point on the discussion as to what defines aesthetic understanding of art. It is a real challenge to be able to say that "This is art" and "This is not." Two people might see the same work sample in different contexts. The common ground that might be found between them is the criteria each uses to define it and being able to articulate what these standards are might be a good starting point as to what determines if a piece of art might actually be a work of art.
This is a really fine distinction, isn't it--the difference between a created piece known as art and a work of art, which rises to another level. I so appreciate tiago's answer, and I sense the passion for art which he expresses. I must confess I'm not an art aficionado. However, I'm an English teacher and this kind of question comes up often in terms of literature--when is it a "classic" piece of writing and when is it just a book. I apply the same standards to music, literature, and art. It has to stand the test of time, it has to reflect some aspect of the human condition, and it has to be written (or played or created) in a way that enhances its meaning. Every old painting and every old statue I see is not a work of art to me, though I do appreciate the longevity and beauty of each piece. However, after many discussions about the arts with all kinds of people, and after sitting at the Louvre for hours over the years and seeing people stop and ponder in awe at artwork I wouldn't look at for more than a moment, I understand that art is much more subjective than literature. While literature has the power to evoke a response in the reader, art is much more powerful in that way because it appeals to more than one sense, and the effect can be overwhelming. That makes every created work a potential work of art, it seems to me.
Well, this is my first answer in enotes =P
The things you see, hear, taste, smell, touch change your emotions, your thoughs, can give you pleasent or unpleasent sensations in your body and, if you are a sinesthet they can provide a great source of enterteinment.
Art can take all those forms. Think of a "piece of art" as something produced by a human with the objective of producing a given reation in another human. If it succedees is a "work of art."
If you accpet that broad definition, that you could define "art" as any form of human work that produces a "feeling, sensation, thoug, etc" inside you that wouldn't be there had you not look at it/touch it/hear it etc, something you want to be there many more times.
In that way, I might look at a painting and feel wonderfully inside, while you could feel absolutly nothing. It would be a work of art for me but not for you. Still it would be a "piece of art" in the sense that it was produced as a way of expression from another human. It is a "work of art" to the person who made it. (assuming he can feel the expression he immortalized there in the first place)
That there's the technical part. Like classical music, once you learn where to focus your atention and what to look for, you can start to see how the "masterpieces" produce a profound inner sensory experience.
Imagine I'm an artist - which I am not lol - suppose I feel a profound sense of gratitude and make a painting about it. My goal would be that other people would be able to see the painting and share the "profound sense of gratitude" I had "stored" within the painting.
Imagine that a given painting takes you to that wonderfull place. You'd be able to pay a lot for it, to have it in your home. Also, you'd considerer it a "masterpiece" and you'd want to see more paintings by that artist because his way of expressing himself trough lines, shadows, shapes and colors really ressonates with you.
ps.: English is not my first language, sorry for any strange gramaticcaly construction =)
pps.: That's how I determine it anyway, other people might have a different opinion. If it causes me to feel a certain way that I enjoy, or if it makes me think a certain way, of it gives me a pleasent emotional experience (be it sadness or hapiness), that it is a work of art, it moves me.