I'm pretty sure it's a window manager for X but is it better than fluxbox or other window managing programs for zaurus pda's?

Yup its just another window manager. Gives you a different type of interface for X. Is it better? Well thats a matter of much debate. It really all comes down to personal preference. Honestly you can tweak the heck out of pdaXrom before you even get to the window manager itself. Personally I'd use the default manager for now untill you're pretty confident you know what you're doing. The only real reason I'd change the window manager would be to free up more memory. Other than that my main focus is getting Snes9x and video playback to work perfectly.

Why not try typing "fvwm" into the search box? You will get many results. In particular this.

http://www.xwinman.org/