WAAS stands for Wide Area Augmentation Spectrum. The European counterpart is called EGNOS (not sure what that stands for). My understanding is that it's a normal GPS signal that has been received by ground-based stations, corrected for additional accuracy, clock drift, etc., and reradiated from the ground station. The concensus seems to be that WAAS signals are accurate to less than 3 meters, whereas normal GPS is somewhere around 7 to 15 meters.
It's not really very good for in-car navigation, since acquiring the signal takes significantly longer than a normal GPS signal. Plus, since it's ground-based, coverage is somewhat spotty.
Prarie Geomatics has an excellent
article on WAAS that goes into a bit more detail.
Here's a
map of WAAS coverage, if you're interested.