Crab legs are the leg and claw sections of crab, sold and cooked separately from the body because they hold most of the meat in large species like king crab, snow crab, and Dungeness crab. King crab in particular has long been harvested in the North Pacific by Indigenous peoples of Alaska and the Russian Far East, and became widely available once commercial crab fishing and processing scaled up in the 20th century.
The meat inside is white to pale pink, sweet, and tender. Crab legs are most often steamed or boiled and served with drawn butter, or served chilled with a dipping sauce; they're also baked, grilled, broiled, sautéed, or fried, and used in crab cakes, bisques, and casseroles. Overcooking turns the meat tough and rubbery, so cooking times should stay short - most crab legs sold retail are already precooked and frozen, and just need reheating.
In the U.S., crab legs are associated with special-occasion dining - seafood buffets, holiday meals, upscale restaurants - while in many Asian cuisines they're worked into hot pot, fried rice, and noodle soups.