Nymphs are the species from which Dryads, Naiads and Mermaids come from.
They are creature of beauty; they don’t actually have a singular appearance, instead you will see beauty when you see them, whatever that is to you.
Every nymph is born a nymph, but they can fall in love and become a dryad if their love is for the green and brown plants of the forest, a naiad if they instead choose to love the blue water in the lakes, waterfalls or rivers; or a mermaid if they fall in love with the sea and its waves.
However, some nymphs never fall in love or never even seek out that love. They remain simply a nymph. Usually these nymphs either live among another society, perhaps human or gnomes. There is even a tale of one who lived with dragons; or perhaps they are so in love with their family that they have no need to leave.