Fish is generally much softer than chicken or beef because it has short fibers and almost no connective tissue. This is why fish cooks very quickly and easily falls apart if overcooked. Fish also has a fresher, lighter flavor, but it depends heavily on its freshness—if it's not fresh, the fishy smell will immediately be noticeable. Simply put, fish can be divided into two broad types: lean fish like snapper or cod, which are low in fat and mild in flavor, and fatty fish like salmon or tuna, which are juicier, savory, and rich in omega-3s.
When looking at the body part, the fillet, or boneless main meat, is the most commonly used because it's practical and easy to cook, perfect for frying, grilling, or sautéing. The belly is usually fattier and has a softer, more savory flavor, often considered the tastiest, especially in fish like salmon or tuna. Meanwhile, the head and bones are often used for stock because they produce a very rich flavor. For whole fish, the part near the bone is usually juicier than the outer part. The key to cooking fish is to maintain the cooking time and temperature—cook it just until the flesh changes color and separates easily (flaky), because if it's cooked too long, the texture will become dry and fall apart.