Probably Japan’s most famous food export, the word Sushi actually refers to the vinegared rice that accompanies the fish. Sliced raw fish on its own is called Sashimi.
Sushi chefs train for years to perfect their skills, and the preparation of Sushi is complicated. Above all, it is important that the fish be extremely fresh. The origins of Sushi are hotly debated, but the vinegared rice was originally a way to preserve the fish. There are variants of Sushi around Japan, but the Nigiri style (the type best-known in the west - slices of raw fish on top of rice) is now found everywhere.
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Species more or less guaranteed to feature in every restaurant are Maguro (tuna), Sake (salmon), Ika (squid), Tako (octopus) and Tamago (egg). More exotic options include Uni (sea urchin roe), Toro (fatty tuna belly - very expensive!) and Shirako (fish sperm).
Most Japanese people eat sushi quite rarely, as it is relatively expensive compared to other restaurants. You can eat sushi with chopsticks or your fingers.