Let’s see if we can make some sense of it all, so you’ll know how to make the choice that’s right for you.
Although manufacturers use different terminology and might vary the specifics a bit, when all is said and done, there are basically three choices in cushions. The first choice is the all-foam cushion. This is usually constructed of a high-density foam core covered by a softer layer of convoluted ("egg crate") foam, and then a polyester fiber wrap, all enveloped in downproof ticking (which prevents feathers from poking through).
This is the cushion of choice if you like a very firm seat and you want the fewest possible wrinkles in the fabric.