This system works differently across airlines. Some have no bidding at all, whilst others have bidding for everything.
Basically, the company generates a list of patterns that need to be flown in the next period (this comes out a month before the start of the period). We then bid for the flying based on a whole bunch of parameters, and a computer program will carefully look at it, and then give you nothing that you asked for.
Some people chase particular flights (for instance across the Pacific, the trips are both short, and high density (i.e. lots of hours for the time away; others drag on forever, and may involve about half the pay per day away from home.)) In my case, I really don't care where I go, my preference is for the trips to start or end in Melbourne.
Relative seniority in the rank/aircraft type decides how you'll fare in the bidding. Some airlines have rotating seniority (in which everyone will be both senior and junior over the course of the year) whilst others use absolute seniority.
