What is the difference between blackout and room darkening curtains?

Blackout curtains block 99 100% of light when a true liner is involved your room stays dark even at noon. Room darkening curtains block 80 95% of light, which is enough to make a bright room dim but not pitch black. If you want to sleep past sunrise or create a home theater atmosphere, you want true blackout (look for sewn liners). For general dimming, room darkening is usually fine and cheaper.