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.
This is what an all-foam cushion looks like in a finished sofa. Notice that there are no wrinkles at all on the seat cushions, and only a few on the back cushions. Back cushions will almost always be of slightly different construction than seat cushions, because they need to be a bit softer, but they still have to be matched to the seat cushion. A softer seat cushion requires a softer back cushion, or the body will jackknife — pitch forward when sitting. This cushion is relatively maintenance free. When you get up from sitting on it, you won’t need to do any fluffing, plumping or primping … it will look just like it did before you sat down.
The second cushion choice is usually called spring down. It is the second-softest cushion and is made with a core of coiled springs, which is surrounded by a foam box and then wrapped with a thick layer of polyester fiber. This is all fitted into a down proof cover filled with a blend of down and polyester fiber. The springs provide a resilient feel, and the little bit of down adds a touch of softness and loft that only down can give.
This gives you an idea of how spring down looks in a finished sofa. It has reasonable shape-holding properties but requires regular fluffing as well as rotation to retain its plump, softly tailored appearance. This is the cushion for you if you like some of the softness of down — a moderate amount of that down squishyness — and don’t mind a little fussing to make it look great.
Down Plush, or Blendown
The third basic choice for a cushion is called down plush, or blendown. It is the softest and most luxurious of the cushions, and is constructed with a thinner foam core wrapped in a thick layer of polyester fiber, and then wrapped in a much thicker layer of a down and poly-fiber blend. Because of its relatively thin foam core and greater proportion of down, it really feels and acts like a pillow.