Katsuobushi (Japanese: 鰹節) is dried, fermented, and smoked skipjack tuna (Katsuwonus pelamis). It is also known as bonito flakes when young bonito is used as a cheaper substitute for skipjack tuna. Katsuobushi or similarly prepared fish is also known as okaka (おかか).
Shaved Katsuobushi and dried kelp – kombu – are the main ingredients of dashi, a broth that forms the basis of many soups (such as miso) and sauces (e.g., soba no tsukejiru) in Japanese cuisine.
Katsuobushi’s distinct umami taste comes from its high inosinic acid content. Traditionally made katsuobushi, known as karebushi, is deliberately fermented with Aspergillus glaucus fungus in order to reduce moisture. Katsuobushi has also been shown to impart a kokumi flavour.
O-nigiri (お握り or 御握り; おにぎり), also known as o-musubi (お結び; おむすび), nigirimeshi (握り飯; にぎりめし), rice ball, is a Japanese food made from white rice formed into triangular or cylindrical shapes and often wrapped in nori (seaweed). Traditionally, an onigiri is filled with pickled ume (umeboshi), salted salmon, katsuobushi, kombu, tarako, or any other salty or sour ingredient as a natural preservative. Most Japanese convenience stores stock their onigiri with various fillings and flavors. There are even specialized shops which only sell onigiri to take out. Due to the popularity of this trend in Japan, onigiri has become a popular staple in Japanese restaurants worldwide.
Despite common misconceptions, onigiri is not a form of sushi and should not be confused with the type of sushi called nigirizushi or simply nigiri. Onigiri is made with plain rice (sometimes lightly salted), while sushi is made of rice with vinegar, sugar and salt. Onigiri makes rice portable and easy to eat as well as preserving it, while sushi originated as a way of preserving fish.
(Information Source: Wikipedia)