A futon is a Japanese cotton mattress. It can be filled with foam, wool, polyester or a combination of fibers. The word "futon" is Japanese, but there are many cultures that have slept on some type of cotton mat. In the west, most futons are sold with fames. They are popular for their versatility, and seem to fit the student lifestyle very well.
The futon become popular in the US until after World War II, about the same time that Japanese started getting into beds. Soldiers returned to America from Japan having slept on them. In the 1960's, futons became associated with the counterculture generation who wanted to break from tradition.
Japanese interior design emphasizes space, light, and natural elements. Fusuma separate rooms, and by rearranging them, owners transform the size, function, and feeling of areas. A traditional Japanese futon set includes shikibuton (under futon), kakebuton (comforters), and makura (pillow). Since Japanese futons do not have frames, they can be stored on the upper shelf in a closet (oshiire) during the day, thus freeing the "bedroom" for other purposes. Tatami floors add another layer of softness to a night's rest on a futon.
Shikibuton are typically stuffed with cotton and wrapped in linen. Japanese change their futon with the season--lighter in summer, heavier in winter. The more expensive down futon are light and extremely comfortable. If you ever get the chance to sleep on (in?) one you'll be delighted. The traditional Japanese pillow (makura) is an odd thing: it is filled with red beans, or buckwheat chaff and makes a lot of noise when you move your head.
Maintenance is key. Hang a futon out in the morning sunshine and it will be less likely to mold. Leave it on the floor all the time and it will soon need replacing, especially in humid Japan. Alternatively, futon can be dried using what's called a "kansouki," a dryer that is placed between the upper kakebuton and the lower shikibuton.
Many Japanese believe that sleeping on a futon on the floor is better for the back than sleeping on a soft bed.