What is the fundamental difference between a polyrhythm and a polymeter? Could you discuss what makes one different from the other?
If you look in a book, the definition of polymeter will probably look something like this:
Whenever two or more independent rhtyhms are played simultaneously, (where one of these rhythms is an irrational rhythm), a polyrhythm is created.
(Irrational rhythms are defined as unequal note groups. Read,  p.22)
However, you asked me to pinpoint the defining qualities of each one, and not to define the term.
In my opinion, a polyrhythm is probably best and most simply described as a "compound tuplet". This compound tuplet’s micro subdivisions are usually made up of rhythmic figures that form part of a smaller tuplet.
A polymeter is best described as the superimposition of one meter on top of another. It could also be described as the simultaneous performance of two or more independent meters.
To sum it up...
POLYRHYTHM = COMPOUND TUPLET
POLYMETER = MORE THAN ONE METER PLAYED AT A TIME
Hopefully these definitions have helped you identify the most basic difference between a polyrhythm and a polymeter. If you would like to learn how to identify, dissect, construct, practice and apply polyrhythms and polymeters step by step, I would highly recommend that you buy a copy.