PBS = Preferential Bidding System.
Instead of the company building lines and having everyone bid on them, the pilots bid for days off, layover cities, day/night flying, long/short legs, or whatever preferences the company comes up with.
After everyone bids the company builds the lines. If you are senior and bid "every Friday off, plus the third Tuesday off and New Orleans layovers" then you will probably get exactly what you want. If you are junior then you get squat, just like you do in any system.
There are people that love PBS because they can simplify their bidding and get the days off/flying that suits their lifestyle. However, PBS is only as good as the company. If your company has a natural tendency to screw over the pilots (ahem...FDX or AA) then PBS will probably not work out as you had hoped. Some people at companies with a better environment love PBS (the former TWA guys in particular).
One thing to consider: with PBS you can say goodbye to conflicting your vacation, training or any sweet carrover conflict deals. When the company builds your schedule you can bet that it will be to maximize your productivity and NOT to maximize your days off. The number I hear tossed around a lot is that PBS (if properly implemented) can result in 10% fewer pilots required to fly the schedule simply becasue of the elimination of trips dropped for vac/training/etc.
Count me as a "no" voter on PBS if it ever comes up at FDX.