The wood woman, or wood wife, is a fertility spirit that lives within forests. In Sweden, she is the skogsfru (or skogsrå)—a woman with a hollow back who couples with men and will sometimes bless their hunting endeavors. In Greece she is the hamadryad, and all the dryad nymphs for the various trees that follow.
It is said that men who couple with a wood woman will waste away, pining for her touch. Wood women will sometimes marry humans, though this usually ends poorly. They are feral spirits, and will eventually return to their forest homes.
Source: Froud, Brian, Good Faeries, Bad Faeries, Simon & Schuster, 1998.