Now-a-days crochet hooks are made of steel, aluminum, wood, and plastic.
The size of a crochet hook is determined by it’s diameter of the shaft. There is no international standardizes size set yet. The metric sizing is mostly used mostly in Europe and the letter/number sizing in the United States. Each country has its own hook sizing. There is an effort on its way to standardize the crochet hook sizes. When in doubt use a metal hook gauge to determine its size.
The steel hooks are used preliminarily for threads. The sizes are from 0.75mm to 3.50mm and/or numbering 14 to 00.
Aluminum, wood, or plastic hooks are used for yarns. Former times crochet hooks were made of an array of materials like: bamboo, bone, brass, horn, ivory, mother of pearl, pewter, silver, tortoise shell, woods. The sizes are from 2.25mm to 15.00mm and/or capital lettering from B to Q.
There are a variety of books on the market that have detailed information on crocheting if you would like to learn more. My preferred reference book is “Encyclopedia of Crochet ” by Donna Kooler with historic background, crocheting terminology and symbols, a visual introduction on crocheting, crochet patterns and projects.