Dryads are the female offspring of the demigod Cenarius, and the sibling-race to the Keepers of the Grove who are the male offspring. Dryads call Moonglade their home as it was where their father would seclude himself.
Dryads can shift their form between the planes to enter the emerald dream at will, allowing them to visit their slain brethren.
In Warcraft, nymph is just another word for dryad, referring to dryads that have gone "a little wild". Usually due to living their whole lives in untamed wilderness like the howling Fjord and Mount Hyjal.

The most notable Dryads are probably lunara_(warcraft) " First Daughter of Cenarius " who features also in Heroes_of_the_Storm and Hearthstone, being the outward face of all Warcraft dryads. And Taryndrella, who suffered a grisly fate.