Nymphs are flies that represent the insect in the nymph stage of the insect life. Nymph patterns are fished under the water surface, usually near the bottom of the stream or lake. Like dry flies, nymphs are tied in both an imitation and generic (attractor) patterns.
Nymph patterns are usually tied with a weighted material to get the fly to sink. They can also be tied with what is called a bead head. A bead head is a bb-sized, metal ball (usually gold colored) that not only helps add weight to the fly, but also is an attractor for the fish.
Here are the most commonly used nymphs:
San Juan Worm