It’s a comforter. A comforter in foreign countries is usually relatively-flat because the room air temperature is stable with the central heating. However, in Japan, there are few homes with central heating, so the type or thickness of futons needs to be changed with the seasons. So a kake-buton filled with the thick cotton battings with the high ability to retain heat is used in winter. On the other hand, a kake-buton which is flat and smooth to the touch is used in summer. A coverlet designed for kake-butons is usually used to keep kake-butons clean.
As an aside, the notation and pronunciation are both “kake futon” and “kake buton”. Even in Japanese, it changes depending on the preceding and following words, dialects, and the rhythm being spoken. However, there is nothing wrong with using either.
In Japanese, both are usually written in kanji, so there is no difference in appearance. (It is distinguished when it is written in hiragana)