There are two different kinds of art, the first tries to sell the viewer something, the second is just art for art sakes. Now, selling the viewer "something" is very broad, this could be a belief, concept, or product. A lot of people would argue that if you are selling a product, then this automatically discounts it as art. Obviously there are differences between selling an item and a belief, but are these do drastic that it means graphic design is not art? In my opinion, no, graphic design is comprised of art.
A lot of art is made to sell an idea, to get people to agree with you, or think in the way that the artist is. Good art challenges what most people think, and makes them reassess. It doesn't necessarily make people change what they believe, but it makes them question it, which is always important.
In general "What is art?" is very broad, there are countless ways art can be created, and this is what keeps it interesting. If artists just kept painting portraits, and that is all that was considered art, then consider how small, and boring the art world would be. This doesn't just have to do with medium though, the craftsmanship of the work can greatly vary. If the craft is lower end, then the concept of the piece should be strong, otherwise this would mean that a preschoolers drawing could be considered art. This isn't to say though that highly technical art cannot have a very strong concept.