Both make bars more comfortable. Hold your clenched fists in front of you, palms down, elbows bent. Now relax your hands and wrists. See how your wrists naturally turn? That's backsweep. Now spread your hands apart, still relaxed, to bar width with your elbows straight. See the arc your hands made? That's what upsweep does for you.

Backsweep makes bars more comfortable sitting or low on your bike. Upsweep does the same thing while standing and cranking.

-Paul