Nymphs are flies designed to imitate insects at their larva state. They are tied with dense materials and on heavy hooks so they descend quickly. The usual way to fish a nymph fly is dead-drifted under a strike indicator, or using a tightline technique that places the fly near the stream bed.
References

The flies from our Classic Nymph Collection comes from Dave Hughes’ 1986 book “The American Fly Tying Manual”. Dave Hughes, well known angler and author of several fly fishing books, presents the basics of tying artificial flies in this excellent, easy-to-understand introduction to a time-honored art form. Hughes covers the basics of equipment, materials, and tying techniques for dry flies, nymphs, and streamers, and includes color plates and anatomy for a variety of the most popular patterns.
The book is available for purchase at many online bookstores.
Classic Nymph Collection
This collection contains several popular and really great looking nymphs, mainly from Dave Hughes’ book The American Fly Tying Manual.