Beginning as a method of preserving fish centuries ago, sushi has evolved into an artful, unique dining experience. In its earliest form, dried fish was placed between two pieces of vinegared rice as a way of making it last. The nori (seaweed) was added later as a way to keep one's fingers from getting sticky.
Technically, the word sushi refers to the rice, but colloquially, the term is used to describe a finger-size piece of raw fish or shellfish on a bed of vinegared rice or simply the consumption or raw fish in the Japanese style (while sushi is not solely a Japanese invention, these days, the Japanese style is considered the de facto serving standard). This can be eaten as is, or is often dipped into shoyu (Japanese soy sauce) and then eaten.
Great care is taken in the creation of the dish and the many methods of preparing the food indicate the importance of appearance to the educated consumer. Sushi is a work of art as much as a food, and while it is now available in a western 'quick and easy' serving style, the traditional ways are far from lost.
There are a few different kinds, depending on how the item is presented. They are:
Usually a bowl or box of sushi rice topped with a variety of sashimi
(How to make chirashi sushi at home)
Aburage (fried pouches of tofu) stuffed with sushi rice.
The little fingers of rice topped with wasabi and a filet of raw or cooked fish or shellfish. Generally the most common form of sushi you will see.
(How to make nigiri sushi at home)
Also called a hand-roll. Cones of sushi rice, fish and vegetables wrapped in seaweed. It is very similar to maki.
(How to make temaki sushi at home)