WAAS (EGNOS in Europe) is a means of improving GPS accuracy to the point where it can be used for a synthetic glideslope on a GPS approach. The GPS signal suffers from variable atmospheric delays which place an upper limit on the 3D accuracy. With WAAS/EGNOS you have a ground GPS receiving station which obviously knows its exact position, and it receives the GPS signal and works out the required correction, which is then transmitted back up to a satellite which sends it back down to all compatible receivers. These then use the correction to produce a very accurate 3D solution, within (of the order of) a metre or so.
I doubt there are any such approaches operating in Europe. However I am not in touch with currently ongoing experiments or proposals.
WAAS/EGNOS is not really relevant to enroute navigation, or lateral navigation on a GPS approach which is already plenty accurate enough.
It's worth having I suppose; my Garmin 496 picks up the EGNOS signal.
As to what a "non baro aided outage" means, I haven't got a clue. The way RAIM works is that if you are receiving less than (I think) 4 satellites, the IFR GPS's baro input (from an encoding altimeter) is used to replace the extra satellite. As soon as that extra satellite is received OK, the baro input is ignored. It might be something to do with that.