Nymphs live in the Emerald Dream and on Azeroth. Unlike the dryad tree spirits, nymphs and other migratory spirits of nature are prospering on Azeroth. They retire in distant places, avoiding the encroaching Scourge, and ignoring the danger it represents. These shallow flighty creatures simply move on when the land they enjoy is tainted by the plague or visited by war — and they have little concept that there will not always be another land, with greener pastures, just over the next horizon. 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.
The frost nymphs of Northrend apparently evolved from the dryads.
Nymphs will be appearing in Cataclysm, located in Mount Hyjal, they're model is currently a frost nymph, it is unknown if it is a placeholder.