Can someone please explain what the "EGR Valve" is, what it does, why I need it on my engine, and how it relates to the PVC valve?

Your education is much appreciated.

Good info! Thanks.

But why is it called a EGR valve? Does it meter the amount of exhaust gas entering the intake or something? EGR stands for...?
i think there is a vacuum line that runs to it, that when vacuum is applied it opens the valve and exhaust gas flows through the metal tubing
yup, by sending in exhast it lowers cyl temps and increases milage. but I took it off my merkur since it is feed through a .125" hole and basicly useless. I only lost 1mpg but your results may vary.

The primary function of EGR is to "heat" the fual/air mixture entering the cylinder. By doing this, the fuel vaporizes more rapidly and into smaller droplets. This promotes more complete combustion and thus improves fuel efficiency plus reduces unwanted hydrocarbon emissions. ... ;)
EGR valves are just that: control valves. They get opened typically by throttle port vacuum (often with a vacuum amplifier because of the large amount of vacuum force required).

When Exhaust Gas is introduced to an already-mixed air/fuel ratio, it has the effect of leaning out the mixture because it adds physical volume to the amount of air. It also robs the combustion process of a small amount of oxygen for the purpose of reducing the peak temperature during the burn. This lowered temperature reduces nirtous oxides in the exhaust.

Under gentle throttle conditions, EGR will improve gas mileage slightly. It will cause lower HP at full throttle unless it drops out, as usually designed. If the vacuum diaphragm in the EGR valve gets leaky (a common problem), the engine will idle roughly and lose top-end power because the exhaust gas is always there. It normally should turn off at idle and high throttle settings.

That's 'the rest of the story'.