Dushko Petrovich's article about "a practical avant-garde" contains this nice passage about what an avant-garde is:
"Fairfield Porter, the great midcentury painter and critic, ... said the avant-garde was always just the people with the most energy."
He also has a nice summary of how the art world works (at least how it worked a couple years ago):
"The scene seemed wild, but there were simple rules all along. You were given a white room in a Big Art City for a month. You had to do something in that room to generate attention beyond that month. You had to be written about, bought, or at least widely discussed. Then you would get to have the white room again for another month, and so on. If you did this enough, you had what was called a career."