Fishing hook sizes are not scaled to a world standard, but the industry comply with the following sizing structure,

32, 30, 28, 26, 24, 22, 20, 18, 16, 14, 12, 11, 10, 8, 6, 4, 2, 1, 1/0, 2/0, 3/0, 4/0, 5/0, 6/0, 7/0, 8/0, 9/0, 10/0, 11/0, 12/0, 13/0, 14/0, 15/0, 16/0, 17/0, 18/0, and 19/0

A size 32 is very, very small just big enough to hold, the 19/0 is a very large hook.

You then get Short, long and regular shank lengths. This is the distance from the bend in the hook to the eye of the hook.

This is then followed by the pattern of the hook,

O’SHAUGHINESSY – This is a standard pattern which is a very strong and heavy forged hook. Good for keeping its shape and ideal for heavy bottom fishing and strong fighting fish.

ABERDEEN – Generally a lighter wire hook. Most commonly used for fresh water fishing and light sea work. Flounder, Mullet etc

CIRCLE – This pattern sets its self in the corner of the fishes mouth and requires little or no strike. If my memory serves these were designed for commercial long lining. They are a very good hook and much under used.

TREBLE – In basic terms 3 hooks attached together. Used most commonly on lures and plugs. Very intense hooking power but not so good for fish with smaller mouths.

DOUBLE – As above minus one