Beginner Portable App FAQ?

Hi, I'm getting my first USB drive and will be running firefox and thunderbird from it while traveling this summer. I am still trying to get my head around using apps from a USB drive though. Is it possible to run any program from a jump drive? Or only apps that are specially programmed? In any case, is there a good FAQ anywhere that I might have missed?

What I am really curious about is a program I use called Wikipad. I know there are wikis like tiddlywiki I could use instead that would probably work on the portable firefox app. Is there a reason I couldn't use this Wikipad program off of a USB drive? Thanks, just a little confused. - franklen