A dryad (; Greek: ?, sing.: ) is a treenymph or tree spirit in Greek mythology. Drys signifies "oak" in Greek, and dryads are specifically the nymphs of oak trees, but the term has come to be used for tree nymphs in general, or human-tree hybrids in fantasy. They were normally considered to be very shy creatures except around the goddess Artemis, who was known to be a friend to most nymphs.
These were nymphs of the laurel trees.
The Maliades, Meliades or Epimelides were nymphs of apple and other fruit trees and the protectors of sheep. The Greek word melas--from which their name derives--means both apple and sheep. Hesperides, the guardians of the golden apples were regarded as these type of dryad.
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. (associated with Oak trees)
The dryads of the ash tree were called the Meliae. The Meliae sisters tended the infant Zeus in Rhea's Cretan cave. Gaea gave birth to the Meliae after being made fertile by the blood of castrated Uranus. The Caryatids were associated with walnut trees.
Some of the individual dryads or hamadryads are:
Atlanteia and Phoebe, two of the many wives or concubines of Danaus
The story "Dear Dryad" (1924) by Oliver Onions features a dryad influencing several romantic couples through history.
The Lev Grossman's The Magicians Trilogy, the character Julia becomes a dryad after having had her shade removed during her rape at the hands of Reynard the Fox. Her transformation accelerates when she visits Fillory in the company of the novel's other principals, and is complete when she and Quentin Coldwater visit Fillory's underworld. 
In the 10th series episode of BBC television series Doctor Who entitled "Knock Knock" the Doctor encounters an alien lifeform similar to wood lice that he dubs "Dryads" due to their nature and relationship to wood.