During the Edo period, "sushi" refered to pickled fish conserved in vinegar. Nowadays sushi can be defined as a dish containing rice which has been prepared with sushi vinegar. There are many different types of sushi. Some popular ones are:
Small rice balls with fish, shellfish, etc. on top. There are countless varieties of nigirizushi, some of the most common ones being tuna, shrimp, eel, squid, octopus and fried egg.
Small cups made of sushi rice and dried seaweed filled with seafood, etc. There are countless varieties of gunkanzushi, some of the most common ones being sea urchin and various kinds of fish eggs.
Sushi rice and seafood, etc. rolled in dried seaweed sheets. There are countless varieties of sushi rolls differing in ingredients and thickness. Sushi rolls prepared "inside out" are very popular outside of Japan, but rarely found in Japan.
Temakizushi (literally: hand rolls) are cones made of nori seaweed and filled with sushi rice, seafood and vegetables.
Oshizushi is pressed sushi, in which the fish is pressed onto the sushi rice in a wooden box. The picture shows trout oshizushi in form of a popular ekiben (train station lunch box).
Inarizushi is a simple and inexpensive type of sushi, in which sushi rice is filled into aburaage (deep fried tofu) bags.
Chirashizushi is a dish in which seafood, mushroom and vegetables are spread over sushi rice. It can resemble domburi with the difference being that chirashizushi uses sushi rice while domburi uses regular, unseasoned rice.