What is the difference between Depron, FFF, Foamboard, EPS, etc?
Let's see if I can get you the basics, and maybe someone smarter than I am can put together a good thread on this for people to link to in the future.
Polystyrene is the actual material the foam is made out of, from there the differences are the result of how it's formed. First of all EPS denotes that it is an expanded foam. Versus XPS type foams which are extruded.
You have two main types of the expanded foam. You have open cell foam, like you're used to with drink coolers. It has the flakey balls that stick to everything when they break. Then you have closed cell foams. These are the denser foams that at least here, are often sold in 4x8 sheets, in thickness from 1/2 inch up to a 2 inch thickness.
Depron is an example of extruded foam, it's what they use in foam board, the kind with the paper on both sides. So is the FFF that you often hear referred to. They differ in thickness and density.
Then you have EPP. This is the abbreviation for Expanded Polypropylene. This is the bouncy foam. This stuff can be bent double on itself and not break. It's great for planes because of this, it's also bad for planes, because of it's lack of rigidity. Often carbon fiber is used to reinforce this foam.
I think that covers the basics. Hope this helps.