Many other types of fish food are also represented by flies, including baitfish, leeches, worms, crustaceans and scuds. So the term "fly" is only generic, and it does not refer specifically to flying insects.
Most flies fit into three basic categories: dry flies, nymphs (sometimes called "wet flies"), and streamers. When you visit a fly shop or browse through a mail-order catalog, you will usually find flies displayed or listed under these general groups.
Dry flies float on the water's surface, and they imitate a wide range of foods, including adult mayflies, caddisflies, midges, grasshoppers, crickets, ants and many others. The magic of seeing a trout, bass or panfish take a dry fly floating on the surface of the water is one of the greatest sights in fly fishing. Deer-hair bass bugs and poppers, used for largemouth and smallmouth bass and panfish, are also dry flies because they float on the surface.
Nymphs and wet flies represent the immature life stages of insects such as mayflies, caddisflies, stoneflies and others. These flies are fished below the water's surface and usually work best just prior to a hatch of water-born insects, when the nymphs and pupae that they represent become active.
Some special nymphs that imitate the emerging adult insect are fished just below or in the water's surface film. Called emergers, these flies are half wet and half dry; but because they don't float on top of the surface film, we'll include them with the nymphs and wet flies.
Streamers are the flies that represent minnows, sculpins, leeches and other swimming food items that provide meals for bass, trout, panfish and saltwater fish such as tarpon, redfish, bonefish and striped bass. Although all fish, regardless of size, will strike streamers, these flies are well known for their ability to take the largest fish in streams, ponds, lakes and saltwater.
Fly sizes are referred to by a number that denotes the size of the hook on which the fly is tied. For most trout flies, the larger the number, the smaller the fly. For example, a size 18 Adams (#18 Adams) is smaller than a size 12 Adams. In sizes smaller than #2, sizes are represented by even numbers, so in our example there is a difference of four hook sizes between the #12 and #18 Adams. Extremely large hook sizes are noted with "ought" sizes, such as 1/0 (called "one-ought" or "one-oh" in fly fishing terminology).
For any hook size, the hook-shank length can be standard, extra-long or extra-short. Extra-long (noted as XL on hook specifications) and extra-short (XS) shanks are given in multiples of standard shank length, such as lXS or 4XL. Hook wire is also available in extra-fine and extra-heavy sizes.
Here is one of the quirks of hook terminology. With regard to size versus the hook number of large hooks with a /0 size designator, the larger the number ahead of the slant bar, the larger the hook. So a size 5/0 hook is larger than a size 2/0. And, these larger hooks increase in size by one number at a time compared to the smaller hooks that are measured only with even numbers (there are hooks in sizes #2/0, #3/0, #4/0, #5/0, etc.).
These bigger hooks are usually used for bass and saltwater flies, but fishermen who stalk really large trout sometimes use these sizes, too.