From what I've seen there's usually a general goal at the start of the campaign. Once that goal is reached the GM will start adding more and more plots and goals to the campaign. For example, one campaign I was watching on a live stream, the players finally managed to defeat the big bad guy. Unfortunately somewhere along the way on their quest to defeat the big bad guy, one of the players got a really nasty curse on them. So there was this whole new arc to the campaign to remove that curse that took them to far off lands and had them traveling through time. It was really interesting to watch.
Word of advice, [censored] is going to hit the fan. Your all players, it's practically a class feature you mess things up.
Edit
As people post in the out of character thread I can then send you a private message with the rules and stuff. So yeh, you can start working on your guys before everyone shows up. I don't have any idea when I might make a cut off day for actual character creation, so don't worry about it right now. http://www.giantitp.com/forums/showthread.php?p=10719570#post10719570
Edit2
New comic day, so it might be a little slow. Doesn't happen often. Well, more accurate to say it happens sporadically. So sometimes more, and sometimes less.