GenCon is a gaming convention previously put on by TSR, and currently put on by Wizards of the Coast since they bought TSR. The main foci of GenCon are role playing games, strategy games, and CCG's. The con lasts 4 days and is usually held during the second week in August, spanning from Thursday to Sunday.
GenCon saw over 20,000 people in 1999, and continues to grow each year. It attracts gamers of all sorts, from the 10-year-old pokemon fanatics to the die-hard Vampire players to the scary-looking nerds to the geeks of all descriptions.
Indeed, you will not only meet all sorts of gamers, but you will also fight with them for a patch of floor space on which to sleep (sleep is a rare and valuable comodity at GenCon), wade through them while trying to get through the exhibit hall, bump into them while waiting in the massive lines for the event registration, sit by them (some of them smell) at various seminars, and converse with them as you vie for world domination in whatever game you happen to be playing at the moment.
While many of the games at the Con are official events, there are far more spontaneous, informal games that go on than there are registered ones. The exhibit hall, which houses something like 200+ vendors' booths, is where you spend all your hard-earned cash on anything from pewter figurines to T-shirts to swords to action figures to art...and, of course, games. There are also numerous seminars, many of which require a small fee and a ticket, on all aspects of gaming, as well as fiction writing, publishing games, art, medieval economics, and on and on.
Thursdays of the Con are usually packed (event registration for each day starts the day before), and you will want to arrive there Wednsday night if possible. Fridays are more packed than Thursdays. Saturdays kind of start to peter off, and not much happens Sunday. If you can only go to the Con for one or two days, definately go on Friday (arrive Thursday night), and Thursday if you can.
The general areas of the Con are open 24 hours, and you will almost always find someone gaming during the odd hours of the night. They tell you no outside food or drink allowed, but clever gamers hide their munchies in their luggage and use the cover of the crowd while eating. Of course, they charge an arm and a leg for the food they have available, but a short walk down the street will usually bring you to some sort of fast food joint.
Unfortunately, the Con will no longer be in Milwaukee because of two things:
One, the state of Wisconsin refuses to reschedule its state fair to some other weekend other than the one in which GenCon is always hosted.
Two, Milwaukee refuses to allow any more hotels to be built in downtown Milwaukee to accomidate the increasingly large influx of people.