Harmonic Balancer
 

Well once again i will start off by saying i know nothing about cars. What is a Harmonic Balancer and what does it do?

The harmonic balancer should be called harmonic dampener.I guess this dates back to the days when there were balance weights on the front end of the crank to help balance the crankshaft.
It's purpose on the 4.6 Ford engine is to dampen the torsional vibrations that come from the pistons being forced up in the cylinder by the crank and being forced back down again by combustion pressure.This causes the crank to twist and unwind just like a spring.The damper allows some of this energy to be dissipated in the form of heat rather than building up and causing a failure of the crankshaft.Building up isn't a good analogy but it kinda softens the shock to the crank.Some people say they aren't needed but the way I see it,If Ford didn't think they were needed they wouldn't have added the expense of the harmonic balancer and would have used a solid crank pulley instead.
If you are thinking of getting Under Drive Pulleys (UDP's) Steeda,March,and ASP (I think) make non piggyback pulleys for the 4.6.The other co's make pulleys that you bolt the old stock balancer to the new pulley to have the benefits of the harmonic balancer.