Pinion angle is the angle between the pinion and the transmission tailshaft. Ideally, this should be 0 degrees (parallel) under all conditions, but suspension compliance makes that difficult. Most folks will set theirs up a bit on the negative side (pinion nose pointing down), as the pinion nose will come up under power and level things out.
U-joint angle is the angle between the driveshaft and either the pinion or the tailshaft (depending on which U-joint you're looking at). This should be less than 5 degrees, but you need some nominal amount of U-joint angle to maintain lubrication of the yoke and cups (there's no relative movement between those two parts with 0 degrees of pinion angle, and eventually the grease will be displaced, metal-on-metal contact will occur, and failure will eventually result).
I hope this answers your questions.