Sushi is a food of Japanese origin made from vinegared rice and various toppings, which are most commonly seafood but can also include meat, vegetables, or eggs. Sushi toppings may be raw, cooked, or marinated. The original term Japanese: 寿司 "sushi" in modern times refers to the rice, not the fish!
The initial procedure was to take the fishes and pack them intact in salt. The enzymes in the digestive system of the fish would proceed to break down the tissue of the fish and produce a slimy smelly substance that is the direct ancestor of fish sauces used throughout so many Asian cuisines. To eat as a side dish required simply scraping away the slimy outer coating.
A fundamental improvement came about when people found a technique using cleaned filets of fish packed in cooked rice. The rice would ferment, and would later be thrown away when the fish was eaten. The result is a very sour fish which still retains the firm consistency of the meat. It rapidly gained in popularity and soon spread through China (where it did not catch on) in the 6th century and into Japan. In Japan it not only became popular, but narezushi can still be found around Lake Biwa, Japan's largest lake near Kyoto.