Crochet hook sizes can be confusing. Unlike knitting needles that are the same diameter from end to end, hooks have so many variables in the shape of the hook and the throat, there's no guarantee that one manufacturer's hook is going to be the same size as another's. Think of the sizes stamped on your hooks as "guidelines" rather than rules...need I remind you that this is a good reason to always check your gauge, and to use the hook that will give you the proper gauge?
The main styles of aluminum hooks are usually referred to by two major brand names: Susan Bates (top) and Boye (bottom). Note the different shaped heads and throats of the hooks. Crocheters usually swear by one or the other. (Personally, I am a Boye girl, but I am not adverse to tucking a few Susan Bates in my bag.)
It's amazing how inexpensive crochet tools are, so it makes sense to buy hooks in lots of different sizes and types so you will always have an appropriate hook for your yarn or project. Aluminum hook sizes in the US start at B (why no size A?), and go up to about K, though crocheters most often use sizes G through J.
Steel hooks (center) are used for crocheting with thread. They are slightly shorter than the aluminum hooks (on right), and have their own sizing system: the larger the number, the smaller the hook. Size 6 is smaller than size 5, and size 14 is downright tiny. And just to make it more confusing, there are sizes 0 and 00, both larger than size 1. The two hooks on the left are Tulip brand--they are more expensive than those you'd find in the big box stores, but they are such a joy to use they are worth it.
Afghan hooks look like a hybrid of a knitting needle and a crochet hook, and it's what you use for tunisian stitches--it's a way of crocheting, but at times there are a number of stitches held on the needle. Some have a hook on each end (see the top two) and the Daisy brand in the center has a cable for holding the extra stitches.
I don't often use wood hooks--aluminum ones are sturdier for my frequent use--but I sometimes find lovely carved hooks that I can't pass up at craft shows. Crochet designer Nancy Nehring teaches how to carve your own crochet hook, and the bottom two hooks are the ones I made in her class.
Don't forget to look for old hooks at thrift shops and yard sales. You never know when you might find a Gem--literally. A US company made a hook with the brand name "Gem." And regularly clean your hooks in warm soapy water, then give them a good polish with a Puffs Plus tissue for smooth, lightning fast crocheting. Try it--you'll thank me!
For more info about standard US and international crochet sizes, here's a link to the Craft Yarn Council of America's excellent resource.