Ok, so I just said (in the title, didn't you read it?) "Real" sf cons, as opposed to most cons for Trek, Dr. Who, etc. What do I mean by this?
Well, most of the latter are commercial events (not all, ok?). You pays your money, gets a ticket, and you gets your day's worth of entertainment. That's it. Nothing at night, and if you pay to come back the next day, you get the same show. The guests get paid for showing up, too, as do the folks who run the convention.
Real sf cons are put on (99.9% of 'em, anyway) by incorporated (or organizations whose cash flow is too low, but fall, by IRS rules, by default, as) 501(3)c not-for-profit educational organizations. Nobody makes money, or at least a living, off of them. If the groups that throw 'em make a profit (sorry, for non-profits, that's a "surplus"), it goes back into a better, or at least bigger, con the next year, or as a cushion, if they have a bad year. Guests don't get paid; neither does the staff. The guests of honor may get transportation, hotel room, and food paid for. The staff that works over a certain amount (9-16 hours, over the course of the weekend) may get either their membership money back, or a free membership next year. Even the committee running it pays their membership fees.
Note, btw, that it's a membership, not a ticket to a show. You are not there to be entertained: unlike many conventions, these are by the fans, from the fans, and for the fans. You, personally, are what it's all about. You're part of the "show", not an "audience"...but more on this, shortly.
What's a con like, in short? The best I've come up with, is that they're a cross between a professional convention, a small town 4th of July, and a giant family reunion, all rolled into one. (gee, 25 words, or less!).
What's a longer description? Well, the membership, usually significantly less than the tickets to the commercial cons, is for the entire weekend; there are no other fees (ok, if there's a con dinner, or such, that may cost more). They crank up at least by Friday night, and don't run down 'til Sunday afternoon. What's going on each day is different than the previous day...and, these days, cons are a four, or five, or six (or more) ring circus. Often, there's the main track of programming, and several side tracks, and a science track, and an art track, and a writing track, and sometimes a costuming track....
There's almost always an art show (most things available by bid, with an art auction later in the con), a dealer's room, with everything from books to sculpture to costumes to edged weaponry to books.
Oh, and there are parties most of the night (with "most" defined as ending somewhere between 3:00 am and 5:00 am). Sometimes there's dancing, or music...but mostly talking til we can't talk anymore. And by the way, the authors, and the editors, and the artists, are all there, roaming the halls, hanging out at the parties...and are accessible: you can talk to them (though if you come up and gush, or make a pain of yourself, they won't want to talk to you, and will find somewhere else to be).
I usually say that, no matter what you want to talk about, at a con party, within 15 minutes, you can find someone who
Sounds a little different that Out There, doesn't it?
Which leads to...what's it all about? For that, see sf fandom.