There are many kinds of flies available that will catch just about anything that swims. On some days fish are deep and a wet fly does the trick, other days especially in low light conditions, floating flies works better. In salt water you can catch Sails on streamers and poppers, and Bonefish on little crab imitations. Fly-fishing is similar in freshwater with dry flies taking Trout on top, and crayfish imitations catching Bass down deep on a stream bed. It pays to carry a wide assortment of flies with you as you never know what the choice of the day might be.
Streamers are used to represent baitfish most of the time. They can be used in saltwater or fresh. Most streamers have heavy eyes tied onto them or in some cases have a lead wire wrapped around the shank of the hook prior to the body material tied on. Streamers are fished in a slow jerk type action almost like a jig. Streams and rivers with current are best suited for the fly because it reduces the action that must be implied by the fisherman. They also work well in a heavy current or tide in saltwater.
Dry flies are designed to be fished floating on the surface like an insect that either fell in the water or hatched from below and rose to the surface. Dry flies are responsible for the whipping action that most people relate to when fly-fishing is brought up. The reason for allot of the waving of the rod and line is to dry the fly in between casts so it will float better. Most dry flies are coated periodically by dipping them in a silicone coating to help water proof the material. Dry flies are made of anything that floats including fur, feathers, cork and wood. Some are left alone on the surface after being cast and some are popped or jerked. Most dry flies are for fresh water fishing but some poppers are used in salt water.
Nymphs are insects like Caddis Pupa or Stoneflies in the pre-flight stage. They need action to catch fish and some even call it an art. The Nymphs have to be cast upstream and let naturally float down with a light touch. The angler has to feel the pickup and strike before the fish feels the weight of the line and drops it. Fishing with Nymphs is one of the hardest forms of fly-fishing.