Nigiri - slices of fish on vinegared rice with wasabi
The word "sushi" is often ambiguous for non-native Japanese. We think of it as being interchangeable with raw fish. Sushi is vinegared rice topped with other ingredients. Sashimi, which is slices of raw fish alone, is not sushi because it isn't accompanied with rice. Originally, sushi was fermented fish with rice preserved in salt, and this was a staple dish in Japan for a thousand years until the Edo Period (1603 to 1868) when contemporary sushi was developed. The word "sushi" means "it's sour," which reflects back to sushi's origins of being preserved in salt.
Traditional sushi took longer to prepare due to the fermenting steps required. Contemporary sushi was developed to be a type of fast food, and remains so to this day.
Sushi in Japan is very simple and usually doesn't contain more than one type of fish or one type of vegetable. In Japan, they don't have the kinds of rolls that are popular at American sushi establishments, such as Rock & Roll, Spider Roll, Caterpillar Roll, and so on. Japanese people also don't put avocado on their sushi. These types of rolls are considered "Western-style" or "California-style." When using condiments, the Japanese keep it simple. They do not mix wasabi in the soy sauce; if they desire more wasabi, they dab a little bit on the sushi. The pickled ginger is intended to be a palate cleanser and is eaten in between pieces of sushi, not with the sushi pieces. Japanese also do not put sauces on their sushi, such as spicy mayonnaise or unagi sauce. To experience sushi as a purist, you want to taste the fresh ocean flavor of the fish, not drown it with soy sauce, wasabi, pickled ginger, or other sauces. Each sushi piece is supposed to be bite-size, but if the piece of sushi is too big, it's fine to take two bites.
Anyway, here are the principal types of sushi:
Nigiri or nigirizushi
An oblong mound of vinegared rice pressed between the palms of the hands, with a little bit of wasabi on top, and a single topping (a vegetable, a slice of tamago (egg omelet,) or a slice of raw fish) draped over it. These are meant to be eaten with your fingers. Place one piece of nigiri between your thumb and fore/index fingers, turn it upside down, and dip the fish in the soy sauce. It is done this way because dipping it rice-side first will cause it to fall apart. Place it in your mouth fish side down.
Maki rolls or makizushi
Cylindrical pieces of vinegared rice and other ingredients wrapped around nori (seaweed sheets,) thin slices of cucumber, soy paper, or thin omelette skin. These are made by rolling the ingredients with a bamboo sushi mat and then slicing the pipe-shaped roll into cylindrical pieces. These are also eaten with your fingers. When the rice is on the outside, like an inside-out roll, this is called uramaki.
See: Make Vegetable Maki With Kids! and Try This: Umeshiso Maki
A pouch of fried tofu filled with rice. It's okay to eat these with your fingers.
Chirashi or chirashizushi
"Scattered sushi" - a bowl of vinegared rice and mixed ingredients. Eaten with chopsticks.
Temaki, or hand roll
A cone-shaped piece with nori on the outside and vinegared rice with ingredients on the inside. Eaten with the fingers.
See: How To Make Chirashi At Home
These types of sushi are still found in Japan, but might be harder to find in the US:
Vinegared rice and ingredients shaped into a block with a wooden mold. This is then cut into bite-sized pieces and eaten with chopsticks.
Still found in Japan, this closely resembles the original form of sushi; narezushi is fermented fish with rice and salt, which is preserved for a few months before being eaten. The rice is discarded after the fermentation process; only the fish is eaten.
(Images: Kathryn Hill)