Quite simply, the purpose a work of art is to exist as art--self-expression that evokes an emotional response from it's audience. The purpose of a design is to

Quite simply, the purpose a work of art is to exist as art--self-expression that evokes an emotional response from it's audience. The purpose of a design is to to perform a specific practical function. That's not to say that design can't be artistic. A Jasper Morrison chair might be incredibly beautiful and expressive, but it's designed to be sat in.