dont worry about it being a dumb question.
sheaths are usually softer (like you said) and could be cordura, leather, and I usually consider kydex and plastic as sheaths. Scabbards are hard, and could be metal or wood, or even a hard leather.
Scabbards are usually for larger blades, but not always, usually because larger blades often entail the knife being "nicer" and ncier knives usually have nicer sheaths/scabbards.
I suppose sheath and scabbard are interchangable, more a matter of preference, but to me, sheath is soft, or plyable, scabbard is hard, and can be used to whack someone in the face