Mochi (餅) is Japanese sticky rice cake used both in savory and sweet dishes.  Mochi is usually made from “sweet rice” (also called Mochi rice and it’s not actually sweet by itself) cooked and pounded until it becomes a paste that is very sticky and smooth, then formed into cakes or blocks.  It is often eaten in New Year’s Ozoni soup or baked with soy sauce.  Mochi made from cooked sweet rice doesn’t stay fresh and soft too long, and usually needs to be grilled or reheated to be eaten later.  Sweet Mochi, however, uses Mochiko (#ad) sweet rice flour and a lot of sugar, and it stays soft for an extended period of time at room temperature because of the large amount of sugar.  It is elastic and malleable, and it can be shaped the way you want for various desserts.  You may even see it at frozen yogurt shops in the US as a topping, which is actually Gyuhi (求肥) cut into mall bite size pieces.