Mochi or omochi is made from pounded sweet rice. Sweet rice is also called glutinous rice, sticky rice, or waxy rice. If you intend to make mochi on your own, you have to find Japanese sweet rice, which is short grain sweet rice.
Traditionally, steamed sweet rice is pounded in a wooden motar with one or two wooden mallets. The entire process used to be a daylong and labor intensive event, which tend to involve the entire community. So that mochi was made only two or three times a year at most.
Since the Japanese consider rice sacred, mochi is even more special. For the New Year’s celebration, kagami mochi, a round shaped double stacked mochi is made as a decoration/offering to gods.
However, factory cut and packed mochi is sold in grocery stores all year round, recently. Also, some Japanese households have electric mochi makers, which resembles electric bread makers.
The shape of mochi depends on the area. People of the eastern part of Japan, which includes the Tokyo area, use a block type of mochi, which is cut from large sheets. In the western area, people eat round mochi. In either case, one piece of mochi is the size of your palm.
Unless freshly made, mochi is cold and hard, so that it needs to be softened by reheating it before eating. An oven toaster works well, but it can be pan fried as well.
The mochi is just plain unflavored rice, so flavoring needs to be added before eating it. The popular and the easiest preparation method is isobe-maki, which is grilling mochi, flavor it with soy sauce, then wrap it in seaweed. It can also be ozoni, which is grilled mochi in soup, or oshiruko, which is grilled mochi in stewed sweet adzuki beans.