What is a nipple driver and would I use one to replace a broken spoke? I have not yet built a wheel (or even replaced a broken spoke), so it's not quite clear to me what a nipple driver is. I also didn't find any reference on Sheldon Brown's Wheelbuilding page.

What is a nipple driver and how is it used? Is it a necessity or a convenience? Would I use one when replacing a broken spoke?
Wanted to add some photos of nipple drivers to augment @mikes's answer. Some are hex-shaft drill mounted drivers (top), some have straight screwdriver handles, others have offset handles to speed wheel building (bottom). They're handy for building up wheels quickly, but you need a proper spoke wrench for truing.