Places to hang out, snatch free food, free drinks, and sometimes to crash for the night if you have nowhere else, though this usually waits for closing, which can be 3 AM or later, and not all suites allow it. Most have theme decoration. Most ask for donations or hold contests. Some allow smoking. Usually this includes some 'rare gems'.
Most shows allow bidding on pieces, and culminate in an auction for pieces with a lot of bids.
Sometimes art with a flat price instead of a bid. The usual format is either a personal performance or, more often, a music circle. Anyhow, a really cheesy con will look like the one in Galaxy Quest. A good one will be quieter, with less emphasis on dealers and tv-show names, and the geek element will be notably lower. Having only been to two cons now, this is how I've gotten most of my info.