I believe art is the expression or application of human skill or imagination, typically in a visual or audial form, primarily for the aesthetic appreciation by another.
That said, there is a certain demand of the artist to understand and apply a knowledge of his craft. Some art is more forgiving of this than others. For example, an elephant's paintings have auctioned at Christie's for high-dollars, while a violinist would be booed off stage if all they could do is scrape the strings.
There is a certain requirement of the artist to learn the rules before having the luxury of deliberately breaking them. Jazz is an obvious example of this.