Originally Posted by FirstClass

Most senior pilot gets what he wants. Most junior gets what's left over.

There is basically two major categories of bidding-

1.) Line bidding - you see a "line" you like for the month, you bid it. The line has all the trips in it for the whole month.
2.) PBS bidding - you build your own "line" by the trip. You choose individual trips and create a line.

Different airlines use different systems based on one of those two options.