Dryads are tree nymphs in Greek mythology. In Greek drys signifies 'oak,' from an Indo-European root derew(o)- meaning '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. They were also sometimes associated with fruit trees.
Myths & legends
"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).
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.
Ash Tree Variants
The dryads of ash trees were called the Meliae. Rhea gave birth to the Meliae after being made fertile by the blood of the castrated Ouranós. The ash-tree sisters tended the infant Zeus in Rhea's Cretan cave.
The Maliades, Meliades or Epimelides ('melas' meaning apple or sheep in Greek) were nymphs of apple and other fruit trees and the protectors of sheep.
Other dryads include the Nymphai Aigeiroi (of black poplars), Ampeloi (of grape vines), Balanis (of the ilex), Karyai (of the hazel-nut), Kraneiai (of cherry-trees), Moreai (of the mulberry), Pteleai (of elm trees), and Sykei (of fig trees).
Normally they are considered to be very shy creatures, except around the goddess Artemis who was known to be a friend to most nymphs.
Celtic mythology variant
In Celtic mythology they called Sidhe Draoi and they are more elf-like and fairy-like Druids rather than bridal tree spirits as they have many males and females.