'White fish' is a culinary catch-all for lean, mild-flavored fish with pale flesh, such as cod, haddock, halibut, pollock, and tilapia; it's a category, not a single species, and shouldn't be confused with 'whitefish' (one word), the specific freshwater fish in the Coregonus genus.
Most white fish used in Western cooking comes from cold-water marine species of the North Atlantic and North Pacific, or from freshwater lakes and rivers. It suits frying, baking, poaching, and grilling, since its mild flavor readily takes on seasoning; classic uses include fish and chips, fish tacos, and chowder, alongside Mediterranean, Scandinavian, and various Asian preparations.
White fish has a delicate, slightly sweet flavor and tender, flaky texture, pairing well with lemon, dill, parsley, thyme, butter, and cream. It's a lean source of protein and, in oilier species like halibut, omega-3 fatty acids.