Sushi is a Japanese specialty of round-grain rice sweetened with rice vinegar, which is slightly sticky and can be shaped in a mould or by rolling. Its flavor is often enhanced with fresh or cooked raw fish or seafood or vegetables. It is sometimes layers in a mould to make rice cakes, wrapped in sheets of nori seaweed and served in slices, or served in bowls. The name "sushi" is actually an abbreviation for "nigirizushi."
Sashimi is an entrée - often the first course in a Japanese meal - consisting of raw fish, shellfish, or mollusks. The fresh fish is trimmed, boned, and cut with precise cutting techniques, according to requirements and/or the type of seafood. The thin slices or strips are then arranged on a plate and garnished with ingredients including daikon, shiso leaves, prickly ash, corkwing, seaweed, and lemon. The world comes from "sashi," meaning "to pierce," and "mi," meaning "flesh." The word is actually not solely for fish, and recipes for chicken and beef sashimi to exist, but are rare.
Both Sushi and Sashimi are often served with dipping sauces like a flavored soy sauce, as well as wasabi, and pickled ginger.
References:
Larousse Gastronomique; 2001; Pages 1041 - 1042, 1168
Food Lover's Companion; 2001; Pages 543, 608
Oxford Companion to Food; 1999; Pages 695, 771