The distinction between pure
art and pure
entertainment is this:
Art is that which challenges standards of conduct or beauty, or attempts to determine why these standards are so. Art encourages thought.
Entertainment is a direct affirmation of these standards, and is encouraging of simple acceptance. It is also not very challenging to the mind.
This isn't to say that art cannot entertain, or that entertainment does not contain artistic elements- this is only an exploration of their pure, nonexistent ideal.