When you have frequently won or have been placed high in the final rankings you have chance to be able to participate in the VGC tournament. This tournament is hard and extremely important. The strongest wifi-battlers compete on these tournaments and are not to be compared with normal wifi-battlers. For entering a wifi-tournament you get a rare berry but only if you have battled atleast one time. On special occasions the winners of the Pokemon wifi-tournaments get a place in the PWT in black/white2. They put them and their teams in the PWT. This is a great honor. You can download them anf battle them when you talk to the lady in green at the PWT.
Hope this made things clear