they do get misused a lot but basically this is what I've learned myself.

poly = plastic

from there is a crapshoot to know if 'poly' is referring to:

polyurethane = the hard as rock and brittle when heated layer.  This is usually the top layers that POP off when you heat gun it.

polyester = the stickier sealer coat usually found underneath most Fender finishes.   Its clear, but when you heat it up, it will yellow just a bit.   Its also the stuff left over when you THINK you're done stripping a finish.   You have to use the dull putty knives to slowly scrape it off once heated.  This is where I tend to scortch bodies by getting them too hot.   

I usually just assume that I'll be dealing with BOTH types when people say, "its a poly fininsh."