Good question by Denali2011 and great answer by rstark....
Here's my comment for what it's worth, notice how I
word this as I'm always
very careful with each word I post on a discussion forum.
Foam pads will finish down more consistently on a wider spectrum of paint systems than any "fiber" pad.
Fiber is and can be used as a mild form of an abrasive and so can foam. But with fiber pads, and it doesn't matter what the
type, (think Wool buffing pads for a rotary buffer, that's a fiber pad, or Surbuf pads, or PFWs), as each of the individual fibers are being pushed against the paint and moved over it in some fashion, in and of themselves they can inflict their own mark.
Foam on the other hand offers a smooth, uniform surface.