The word "futon" comes from Japan and means "bedding". The Japanese use the word "shikibuton" to describe the actual mattress portion of the bedding -- the part we now call "futon".
Japanese futons are quite a bit different than what we are used to in America. They are two to three inches thick and are made primarily of long staple cotton, the most expensive part of the cotton ball; long staple cotton is the most valuable part of the cotton ball because it is usually spun into thread to make clothing. This long staple cotton doesn't have as much loft (i.e. isn't very fluffy) as other parts of the cotton ball, but it has strength and integrity, so the cotton batting in the mattress won't pull apart and get lumpy.
The Japanese traditionally roll out their futons on their floors every night to sleep, and roll them back up in the morning because space is such a premium in their densely populated country.