Although there are hundreds of types of flies used in fly fishing, most of them fall into five specific categories or types. These types are dry flies, drowned flies, nymphs, streamers and bucktails, and terrestrials. The fly is essentially used to imitate an insect that the fish is trying to eat.
A dry fly imitates a natural insect that floats on the surface of the water. Fish are very sensitive to any movement in their water and how the currents move the insects they need to feed on. In fly fishing, if a dry fly moves even slightly against the current, the fish will have nothing to do with it. The fly may look like something the fish recognizes, but it doesn't act the same way as an insect. The fish recognizes it as something foreign in the water and leaves it alone.
In fly fishing, a drowned fly imitates a naturally drowned or drowning insect and is fished below the water's surface. No one knows if the drowned fly is considered a drowning adult insect or a nymph from the fish's perspective. Most fly fishermen today seem to believe that it is considered a nymph. For this reason, fewer and fewer drowned flies are being sold. Drowned fly fishing is the oldest form of fly fishing. It dates back to the descriptions of the early Macedonian peoples.
A nymph is a stage between an egg and an adult in the life cycle of an insect. In fly fishing, flies that resemble nymphs are gaining in popularity. The nymph fly is just below the surface of the water. When a fish swells the water without breaking through the surface, it nymphs. This means that the fish eats the natural nymphs just as they emerge from their shells. This is what a nymph fly mimics.
Streamers and bucktail flies do not imitate any part of the insect's life cycle. These types of fly fishing flies are much larger and represent small baitfish such as sculpins. The main difference between these two types of flies is that streamers are tied with feathers and bucktails are tied entirely with hair. Fly fishing using these two types of flies generally requires more rod and line manipulation. The movements are meant to mimic the movements of small fish.
While most flies represent aquatic insects, a terrestrial fly is designed to imitate a land insect that has fallen into the water. The two most common terrestrials that are imitated for fly fishing are the ant and the grasshopper.
In addition to these five basic categories of flies, there are many other types of flies that are used for fly fishing. Some of them are a combination of one or more of the basic categories and some do not fit into any group. The most important thing to remember is that it doesn't matter what the fly looks like to you, the angler. It matters how the fly looks to the fish.