Well, first off, unless my google-fu is weak, it has been cancelled:
http://forum.genconoz.com.au/viewtopic.php?f=1&t=850
However, at least in the states, GenCon is a gaming convention. It's a gathering of gamers of wargames, boardgames,
RPG's,
CCG's, etc, etc. Lots of companies will announce releases, hold demos and generally smooze with the public. There are also a lot of 3rd party vendors who have discount prices on a lot of various gaming related items (miniatures, sourcebooks, paints, you name it).