Forced reps are using help (either someone elses, or your own on exercises that make it possible) to get the weight up once you have reached the point where you can't do another rep in good form. It is often combined with negative reps: help on the positive lift, but using your own strength to resist on the negative (this usually causes more DOMS). You can do forced reps on dumbbell bicep curls, for example, by using your other hand to help you lift the weight.
Pyramidding is increasing weights with each set, while at the same time decreasing reps. Going "up" like this accustoms the muscle to lift a heavier and heavier load, kind of like the first set is a warm-up, then the second set is a warm-up for the third, etc.
I recently read about a bodybuilder who does his own version of pyramidding (that looks like awfully strange spelling, doesn't it?): he would increase the reps with each set, but keep the weight the same.
You can pyramid down (and Cathe offer this option as a pre-mix on the Pyramids DVD), but I don't recall seeing this recommended in any of the books I've read, except as the second half of a full pyramid (going up, then going back down in weight).
You might be able to do more reps going down, but the number of reps in a workout is only one factor that contributes to intensity.
