As always, thanks to Chris for his reference...
As with almost anything airplane related, there's lots of misunderstanding. The fuel system on transport category airplanes is at the top of the list. In fact, during an examination for a Type Rating for a new pilot in command on large aircraft (more than 12,500 pounds) an inordinate amount of time is spent by the candidate explaining his/her knowledge of this system
The aircraft can be both fueled and defueled from the ground. On most transport category aircraft the "single point" fueling system is accessed through a panel on the underside of the leading edge of the right wing. Simply stated, a valving system and fuel quantity gauges (replicating the ones in the cockpit) are located at that site enabling the fueler to add or subtract fuel.
A relatively easy and quick process.
Fuel is the most easily manipulated weight in the aircraft... up to the points to which Chris and Ankou allude.
Enough fuel is required to get to the destination, make an instrumet approach, miss it and fly to the alternate airport declared in the flight plan as well as fly for another hour after that. All pilots fudge that to the "fat" side, if possible.
A word about flying with too much fuel.. It's deleterious to performance (and therefore, expensive) to "tanker" fuel. Weight equals lack of perfomance and more cost so the Dispatch Department will begin to question the Captains addition of reserve fuel after a certain point... Constant fight with those guys...