A fly is an artificial fish attractor made out of materials such as synthetic fibers, feathers and animal fur used to imitate a specific food source. The materials are attached to a hook in a manor that will assimilate the fishes diet. . Most flies are tied to match certain stages in an insect's life. Some flies are tied in a way that does not imitate the natural food source but rather to trigger the predatory instincts of the fish. A leader is a piece of clear monofilament that is attached to the end of your fly line.
Leaders can vary in length and diameter depending on the fishing situation.
They usually are tapered to accommodate the type of water, fish and fly. The way it is tapered affects the manor in which the fly is presented to the fish. You need leaders because the end of a fly line is too large to attach a fly and present it to the fish in an effective way. A leader is clear and therefore difficult for the fish to detect. Each time you remove a fly or get snagged and the fly breaks off, a section of your leader is lost.