Nymphs are minor female deities that preside over various elements of the natural world. Classically, they focus on a particular locale, such as a copse of trees or branch of a river, enjoying a supernatural rapport with and control over their aligned element; this rapport and control is typically used for the betterment of the nymph's domain, though violent defense via elements supernaturally animated and controlled is not unknown.
The classical variations of nymphs are as follows: Dryads (forests), Naiads (springs and rivers), Nereid (the Mediterranean), Oceanids (the sea) and Oreads (mountains), Limoniads (meadows), Limniads (lakes, marshes and swamps) and Napaea (valleys). In a broader milieu than that of Greek mythology, other varieties of nymph are not only likely, but inevitable; nymphs of vacuum or plasma, or patrons of byways in cities that have been allowed to grow along organic lines, are only a few examples.
Nymphs dedicated to more abstract concepts, like philosophy or poetry, are technically termed Muses. Ultimately, the difference is negligible; the pursuit of those concepts is as natural as thought, after all.
Being spirits, their physically manifested forms are somewhat variable, though typically the appear as nubile women, marked to some degree by the element or concept they are associated with. A nymph that presides over a stand of oak trees might express a crown of leaves instead of hair, or skin patterns reminiscent of wood grain, for example.