Zafu is a zabuton (Japanese cushions used to sit on the floor) used for zazen (Zen sitting meditation). There are two kinds of zafu, circle shaped zafu of the Soto sect and rectangular shaped zafu of the Rinzai sect.
The origin of the term "zafu" (literally, bulrush for sitting) was that when this cushion for zazen begun to be used actually it was made from bulrush fluff. At present days in the beginning of the 21st century, it is not made from bulrush fluff in Japan or China, but cotton, kapok, or buckwheat hull is used for inner material and velvet or cotton is used for outer material.
About circle shaped zafu
The width is 35cm and the height is 20cm, it is made of three cuts of cloth with color of black, dark blue and brown (for upper, bottom and surrounding) seamed together.
A zabuton (76cm x 71cm) may be used underneath the zafu, but generally it is only for a leader.