GenCon 2007 was my first (and so far, only) convention.
There is a "general" admission price and that buys you your GenCon pass. This gets you into the Con and allows you to roam pretty freely throughout the facility, including the Exhibitors Hall. Passes this year looks like they will be $68 for a Four-Day Pass (there are other alternatives, such as Saturday-only). Bit of advice: buy the pass in advance. I stood in line for a long time to buy mine the first day.
Registration opens February 2nd.
Everyone that has a pass can pick up a "swag bag" - usually filled with all sorts of things from some of companies. Can't remember what all I got in mine, but there was quite a bit.
If you want to attend a painting class, enter into the painting competition, etc, then you'll need to buy Event Tickets.
I carried my shoulder briefcase everywhere I went the whole four days I was there, so I don't think they worry about things like that. Can't remember if my bag was searched upon entering, though.
Link to the GenCon home page: http://www.gencon.com/