Capacitor upgrade jargon definitions

What is "poly" ?  -STYRENE or -PROPYLENE?  Which is available as the least cost replacement for bipolar electrolytic radials, 100V 8 and 15uf ?  Are "poly" caps foil, or film, or foil AND film construction?  People use "poly" and "film" interchangeably as though they are the same thing.  Are they?  And lastly, are "poly" caps non-/bi-polar ?  If not, how do you know which way they go in?  Thank you.