If you're making sushi at home, you're free to put whatever you like in your sushi. However, certain ingredients are considered "classics," due to the traditions and tastes of Japanese cuisine.
Although raw fish isn't required, some of the best sushi is made with this ingredient. Saltwater fish are less prone to bacteria and parasites than freshwater fish. Keep in mind that species and nomenclature differs between Japan and the United States, and that similar varieties are often substituted for each other depending on location and the season.
Several varieties of tuna are among the most famous sushi ingredients, including bluefin, big-eye and yellow fin tuna. The higher the fat content, the more prized the meat, with the belly meat of the bluefin tuna, known as toro, at the top of the list. Raw tuna meat comes in varying shades of pink, and has a rich, almost buttery flavor. Mackerel is also common. Salmon, caught at sea, is a sushi favorite. Raw, the flesh is a startling orange color, and it has a strong flavor. Other seafood sushi toppings include shrimp, squid, octopus, eel, clams, and roe (fresh fish eggs).
Tamago is a specially prepared omelet made by adding thin layers of egg until they form a dense, inch-thick slice. This is then fastened to a morsel of sushi rice with a band of nori. Avocado is a favorite ingredient in Japan, where its name roughly means "tuna of the land." Cucumbers and mushrooms are also high on the list of non-seafood sushi ingredients, but you can use virtually any vegetable.
Some of the most important sushi ingredients aren't actually part of the sushi at all. Shoyu, a type of soy sauce, is used for dipping pieces of sushi. Eating pickled ginger, or gari, cleanses the palate between sushi servings. Wasabi, a pungent green cousin of horseradish, is made into a paste and either used directly in sushi or added just before eating for a little extra kick. Real wasabi is rare in the United States -- often a horseradish and mustard paste (dyed green) is used as a substitute.