What is art? What makes art?
The answer to this question has changed a lot over history, so it depends on when it is being asked. To the ancient Greeks art was what captured anything that was considered "beautiful" about the human form or organic world. That is no longer the case today. Today art is anything that turns something abstract, like an emotion, which has no shape or color or form but is something felt, into a physical thing, like a painting, or anything that questions what art is.
I had a professor in college who said that "it is the job of the philosopher to define what art is and it is the job of the artist to break that definition," meaning, art's job is to redefine what art is, so you could say that art is anything that redefines what art is in its historical moment. That could be as simple as painting something that has never been painted before, or as complex as something like Duchamp's Readymades, which forces us to ask, if a urinal is put in an art gallery, does that make it art? It's not this specific question but the fact that the object is forcing us to ask questions about art that makes the object art, because it is attempting to redefine what art is.
The goal of postmodern art has always been to produce art that questions what art is, but art in previous periods had different goals, mainly to put into form abstract things like emotions. So today art can be anything that does either of those things.