A nymph fly is fished while it is submerged below the surface of the water. Therefore nymphs are created to look like different insects that are in the immature stage of their life when they live under the water. As a result a nymph fly is usually very successful because trout eat underwater approximately 80% of the time!
In addition a strike indicator is often used, because the fly is underwater and you cannot see it. Therefore an indicator is attached to your fly line and floats on the surface of the water. As a result when a fish strikes and attempts to eat your nymph, the indicator will be pulled sideways or underwater so you know when to set your hook.