(This is quite hard because wingtip has two syllables. :P Also I think my explanation is more confusing with only one syllable words.)

Here is a pic

https://upload.wikimedia.org/wikiped..._and_AB319.JPG

The red one (TK) is the wing tip thing and the not-red one not as close (4U plane) has the wing fence.

When you have a plane the air on the wing rolls on the wing in rolls, as well as flow over it straight. The rolls slide off the end of the wing. These rolls are still stuck to the air on the wing, even though plane now gone fast and not with that air now. This air left by the wing now drags and slows down the plane. This is bad for fuel use since the plane must burn more fuel, since now we must drag far-gone-air with plane too.

Good pic shows these rolls of air from sharp edge of plane: https://en.wikipedia.org/wiki/File:F..._wingtip_1.jpg The rolls turn white when the air get cold and the wet come out of the air to make small drops. (This plane in the pic fast but not as fast as the good fast plane we all love but now gone. )

But the wing fence or wing-tip thing stops the rolls of air from fall off so well. So less far-gone-air drag. Of course wing fence must be made to flow in the air well with the plane or it is worse than no wing thing, but the way it is put to stop the rolls of air which fall off makes far-away-roll-air drag less bad.

(https://en.wikipedia.org/wiki/Wingtip_vortices)