Despite the seemingly contradiction, there are a lot of pieces under the title "Concerto for Orchestra". (Unlike in a traditional concerto that features one solo instrument, a Concerto for Orchestra treats the entire orchestra as the soloist.) I have fourteen in my iTunes library, and know (or at least know of) several more. It's hard to say much about them as a genre — each one is pretty unique — but they do tend to be virtuosic on every level, displaying compositional dexterity and orchestrational brilliance, and calling for impressive feats from all the players, both individually and as a group. Higdon's fits right into this mold; it's a tour de force from all involved.