The Nymphai (or Nymphs in English) were minor nature goddesses which populated the cosmos. Although they were ranked below the gods, they were still invited to attend the assemblies of the gods on Olympos.
The Nymphai presided over various natural phenomena: from springs, to clouds, trees, caverns, meadows, and beaches. They were responsible chiefly for the care of the plants and animals of their domain, and as such were closely associated with the prime gods of nature such as Hermes, Dionysos, Artemis, Poseidon and Demeter. (from http://www.theoi.com/Cat_Nymphai.html)
Another interesting article can be found here:
Guardians of the Land, of all Nature,
Nymphs, Daughters, Maidens,
Know that you are remembered & revered.
Know that you are beloved.
Accept these offerings truly given by
A Daughter of the Old Ways.
Hear me and know my name.
(E A Kaufman, copyright 2008)