I was at the bookstore this afternoon. I was just browsing when I came upon the art section. For some reason, architecture and design (interior, fashion, graphic, etc.) books were also there. I pondered on this afterwards. I am an interior designer by profession and all I can say is that they are different. The two are very closely related, crossing each other's territory even, but different nonetheless. In my opinion, there are two factors differentiating the two money and function.
Being an interior designer, I have to please my clients by adjusting my designs to their tastes. Sometimes, no matter how much I dislike my clients' visions, I have to make them [the visions] come true because I am paid to do so. All I can do is give them suggestions, since interior design is a profession dealing with designing of residential or commercial interiors customized to suit the budget and needs of the user(s) and the function of the project without sacrificing its aesthetic beauty, which can be achieved through thorough research and analysis, careful planning, considerate and practical selection of resources, and accurate implementation. What am I trying to say? In short, design, unlike art, is driven by money.
In art, the artist usually doesn't care whether or not he or she sells his or her work. As long as he or she expresses himself or herself in the artwork, everything is just fine. If the artist gets commissioned, however, it is still considered art, but if the client dictates to the artist what the artwork should look like, then it is considered design.
Now, to be able to sell your design, it must have the function your client needs. In interior design and architecture, we usually follow Louis Sullivan's dictum, "form follows function." For example, if I were hired to design a café, I wouldn't make it look like a library. Unlike art, design must always be practical and must always fit the requirements of the client as they are paying you to comply with them [requirements].
Let me summarize the last four paragraphs. Money + Function = Design; Freedom of Self Expression = Art.
What I just wrote above is just my opinion on this
issue. Just in case you have any comments on it, I would be more than willing to listen.