To add to this, WAAS uses a combination of ground based equipment and geostationary satellites.

The ground based equipment is responsible for detecting the necessary corrections, which are then sent up to the WAAS satellites which will in turn broadcast those corrections.

Unlike GPS satellites, WAAS satellites are always located over North America due to the fact that they are in a much higher, geostationary orbit.

Edit - WAAS also provides GPS-based vertical navigation (e.g. descent profile in an approach) - IIRC this is due to the fact that the ground station transmit accurate atmospheric pressure readings up to the geostationary satellites, then the GPS receiver in the aircraft can use those to adjust altimeter readings. GPS-based vertical navigation is a big deal, because many airports don't have systems such as ILS.