What's the difference between blackout and room-darkening curtains?

Blackout curtains use a multi-layer construction (often with a sewn-in black liner or coated backing) to block nearly all light, while room-darkening curtains just use heavier fabric to dim the room. Specialist subreddit threads repeatedly warn that many products labeled "blackout" on Amazon and Target are actually room-darkening — if the panel is thin and lightweight, it's almost certainly the latter.