Personifications of the more pleasant aspects of nature, nymphs were a kind of middle being between gods and men, communicating with both, loved and respected by both.
They were gifted with the power to make themselves visible or invisible at will, they could perform many feats normally only possible to the gods, and they were always young, beautiful and happy, carefree and loving, and other good things. When they were entrusted with the care of a thing or person, as in the case of Ida and Althaea, who were charged with looking after the infant Zeus, they took this responsibility very seriously, also showing the practical, able side of female nature.

There were many different classes of nymphs -- different ones for rivers, mountains, trees, and so on -- but they were universally female and beautiful. They lived, like the gods, on ambrosia, but seldom visited Olympus, preferring instead to remain in their secluded grottoes and forests, mountains and rivers, trees and lakes, etc. They accompanied many of the gods when the deities walked the Earth, in particular Apollo, Hermes, Artemis and Dionysos, but they were always hostile to the wanton Satyrs.

The twelve main classes of nymphs were:
Dryads or Hamadryads (nymphs of woods and trees)
Oreads (nymphs of mountains)
Limoniads (nymphs of meadows and flowers)
Napaeae or Auloniads (nymphs of mountain vales)
Okeanids(nymphs of fountains and streams)
Nereides (nymphs of the sea)
Naiads (nymphs of the liquid element)
Potamids (nymphs of rivers)
Limnads (nymphs of lakes,marshes and swamps---these were the only evil nymphs)
Pleiads (seven daughters of Atlas and Pleione)
Atlantids (daughters of Atlas)
Hyads (daughters of Atlas and Aethra).