Learn the terms of the trade.
Chef Resources put together a substantial list of terms and definitions that get used a lot in the industry. For more terms and definition visit Chef Resources.
Whole Fish -or- Round Fish -or- Fish in the Round
Whole head on with viscera (guts), tail, everything intact.
Whole head on, tail, everything except that it has the viscera (guts) removed.
This is a Dressed fish which has its fins, tail, and head removed.
H&G (Headed & Gutted) Fish
Whole, head-off, gutted.
Bullets -or- Rounds
H&G with fins and tail removed.
Top Back Loin
Taken from larger fish like Tuna, Swordfish, etc, this is the back loin without the belly portion. No bones.
This is the prime part of a fillet from a large round fish. It is the part of the fillet which is above the spine, sometimes called the top back loin. Loins are typically cut from fish like Tuna, Swordfish, and Marlin.
A fillet is an entire side of a fish with the backbone out. Round fish yield two fillets (one from each side). Flat fish yield 4 fillets (2 from each side). Fillets from larger fish can be further portioned into supremes or escalopes. They may be skin-on or skin-off.
Steak -or- Darne
Is a thick, cross-section cut from a round fish, perpendicular to the spine. Steaks often retain part of the backbone.
This is a steak-cut (bone-in) from a flatfish such as flounder, halibut, sole, or turbot.
Used with small fish like trout, sardines, mackerel or herring, this cut is made by leaving the two fillets attached by the skin. Thus you get the whole fish minus the head and rib bones.
Same as a butterfly fillet except that the head is left on.