In the broadest sense "seafood" is any water-dwelling edible, the water being fresh or salty. It thus encompasses fish, shellfish and seaweeds. Fish are defined as cold-blooded aquatic vertebrates possessing fins and gills. Shellfish, aquatic invertebrates whose outer covering is a shell, comprise crustaceans, characterized by jointed limbs, segmented bodies and exoskeletons, and molluscs, variously gastropods (single shelled), bihalves (with double-hinged shells), or cephalopods (mostly with a modified internal shell). Seaweeds are plants: marine algae distinguished by their coloration.

Within these groups are a multitude of species. The names by which many are popularly known are confusing; different species are given the same name and this is compounded by there being a range of names for the same seafood. To identify a seafood, the principal alternative vernacular names, along with the Latin, are listed.

Names notwithstanding, fish can be divided into categpries according to their size, shape, flesh density and fat content. For the cook, these characteristics are what matter; within a category one fish can be substituted for another.

Even with increasing aquaculture, much seafood consumed is wild. Therefore seafoods of the same species will not necessarily be uniform in flavour, as their habitat, and thus their diet, vary. For safety's sake, even more than flavour, it is important to be sure of their source.

Generally, fish is best eaten very fresh. Being cold-blooded, fish have enzymes effective at the temperature of their home waters; refrigerated, they continue to decompose. To slow this process, fish were dried, salted, pickled and smoked. Despite modern methods of preservation, many traditional methods are still practised for the flavour they impart.

Typically seafoods need little cooking - just until the muscle protein coagulates and becomes opaque; with fragile connective tissue, short muscle fibres and relatively low fat, they become dry and toughen or disintegrate if cooked beyond that point. Very fresh fish and shellfish may be eaten raw.