It is quite difficult to define what art is. This is true for many abstract nouns. Here are some examples from the first 'google-results' page:
art n. Is an inspiration, a form of thinking,a piece of your mind which makes your imagination play with colours, shape and reality. If an art object does not have an idea inside it does not speak to you it is dead.
art /ˈɑɚt/: An activity that is done to create something beautiful or to express important ideas or feelings.
art mass noun The expression or application of human creative skill and imagination, typically in a visual form such as painting or sculpture, producing works to be appreciated primarily for their beauty or emotional power.
art the conscious use of skill and creative imagination especially in the production of aesthetic objects
art is a diverse range of human activities in creating visual, auditory or performing artifacts (artworks), expressing the author's imaginative or technical skill, intended to be appreciated for their beauty or emotional power
art noun the quality, production, expression, or realm, according to aesthetic principles, of what is beautiful, appealing, or of more than ordinary significance.
This definition which I found on Kunstgeschichte-online.com (an art history site) simplifies things a bit:
'Art is what you consider to be art'. ;-)