What is WAAS?

And while we are at it, what makes these OEM units worth the extra $50-$100? It seems like without the screen, buttons, etc then should be super cheap, but they are more expensive than my eTrex which to me seems like a pretty good unit. I assume they are not significantly more accurate since the eTrex claims accuracy within about 20 feet most of the time. So what are the extra features that increase the cost and are they useful for an in car application?

-Mike