The term nymph is interchangeable with dryad. However, there are unique characteristics affiliated with each term. To be specific, the term dryad refers to the forest entities that are a formal part of the "Cenarion family." Nymphs, on the other hand, are considered to be a wilder variation.
Some nymphs have joined the Alliance because of their ties to the night elves.
This section contains information from the Warcraft RPG which is considered non-canon.
Nymphs, along with the more powerful corrupted nymphs, are a type of creature that lives in the Emerald Dream. Some nymphs have been corrupted by demonic influence to become tainted nymphs.
The nymphs are fully described in Appendix Three of Manual of Monsters, however, the validity of the source is disputed.