Dont kill that oak tree!
Nymphs are the daughters of Gaia. They inhabit all natural places. They take on various forms but not all of them are immortal. If a Nymph says that she is an Hamadryad she is saying that she lives in a oak tree and that when the tree dies, she dies also. If she says that shes a Dryad she is immortal, like her sisters the Naiads and the Oreads. Naiads inhabit lakes, rivers and (especially) springs. Oreads live in the mountains.
This Fun Fact was taken from The Oxford Companion to Classical Literature.