Dryads are tree nymphs in Greek mythology. In Greek drys signifies 'oak,' from an Indo-European root *derew(o)- 'tree' or 'wood'. Thus dryads are specifically the nymphs of oak trees, though the term has come to be used for all tree nymphs in general. "Such deities are very much overshadowed by the divine figures defined through poetry and cult," Walter Burkert remarked of Greek nature deities (Burkert 1986, p. 174). Normally considered to be very shy creatures, except around the goddess Artemis who was known to be a friend to most nymphs.
The dryads of ash trees were called the Meliai. The ash-tree sisters tended the infant Zeus in Rhea's Cretan cave. Rhea gave birth to the Meliai after being made fertile by the blood of the castrated Ouranos. They were also sometimes associated with fruit trees.
Dryads, like all nymphs, were supernaturally long-lived and tied to their homes, but some were a step beyond most nymphs. These were the hamadryads who were an integral part of their trees, such that if the tree died, the hamadryad associated with it died as well. For these reasons, dryads and the Greek gods punished any mortals who harmed trees without first propitiating the tree-nymphs.