I'm jumping in here. upB walljump is a very useful move if you're on the stages it's possible on (I believe it's just Yoshi's Island, and Final Destination, but there may be a third stage) upB walljump is this:
Walljumping after doing your upB as a recovery(or whatever else, but that's usually the use) and then doing whatever else you want to do. I personally use another reverse upB onto the stage, but I suppose once I get better, I may start doing some attacks before doing the upB.
Ok, it can ONLY be done UNDER THESE CONDITIONS:
1. You MUST start the upB while hugging a wall(that's why it's so limited in the stage options)
2. You MUST end the upB still hugging the wall
3. IMMEDIATELY hit the direction away from the stage at the peak of your upB.
This is a nice tech to learn, but it can be tough. Once you get the timing down consistently, it's a fairly easy thing to do. Try slowing down the speed in Training Mode.
Everything else Tujex said I concur with.