Fish are classified in different ways, according to the type of flesh –

White fish has a distinctive white flesh and includes such favorites as cod, sole and sea bass.

Oily fish has oil distributed through its flesh, giving it a gray or red tinge. This group includes trout, mackerel and sardines.
Shellfish: this group includes crustaceans and mollusks, the group with shells.

They are also classified into groups determined by their shape

Round fish have a rounded body with eyes at either side of the head and always swim dorsal fin up. This is a very large group which includes a whole range of fish from freshwater salmon to sharks.

Flat Fish have both eyes on the top of their head, they swim on their sides; they are also quick and easy to cook. This group includes skate and flounder.

Fresh Fish

Fish is best when it is fresh. It is best to eat fish the day that it is bought. When buying fish, one must shop around in order to be certain that the freshest individuals are bought. Fresh fish have shiny skin with a metallic glint and they are covered with a transparent mucus, this becomes opaque when the fish is old and stale. The eyes should be clear, bright, and slightly bulging. The flesh should feel firm and springly when you press it with your finger. Stale fish has an unpleasant fishy smell, the skin looks dull and unattractive and the eyes are sunken.

A great way to test for freshness is to open the gills; they should be a clear red or rosy color not a dull brown. Look for scales too; a fish that has been dead for a long time will start to shed its scales. Readily prepared fillets should be neatly trimmed with mist, firm, and translucent flesh. It is advantageous to buy whole fish as it is usually cheaper than prepared cuts, and it is easier to check for freshness. If you are not able to use the fish immediately, keep it in the refrigerator overnight in a sealed container.

However, it is not always possible (or convenient) to buy fresh fish, although nt as potent, there are plenty of other options available.

Frozen Fish

Fresh fish is seasonal but frozen fish is available year round. Frozen fish is not a bad option, as the fish are frozen as soon as they are caught to preserve freshness. The fish is also almost always prepped for cooking before it’s frozen. This makes frozen fish particularly easy to deal with. White fish may be stored in a domestic freezer for up to four months, whereas oily fish should be eaten after three months.

Canned Fish
Canned fish is a wonderfully convenient method of acquiring fish. Oily fish in particular is great for canning and is useful for many recipes.

Smoked fish
Store bought smoked fish is largely available. Smoking is a method of preserving the fish, and results in a characteristic smoky flavor. Kippers are probably the more well known of the smoked fish types available. Smoked mackerel is also widely popular, as well as smoked salmon.