Nymphs were Narnian spirits of nature that lived in the trees and waters. They were capable of leaving their tree and watery domains and assuming a physical body resembling human form.
There were two types main of nymph; Dryads who dwelt in the trees, and Naiads who dwelt in the rivers, streams and wells. They were so spiritually tied to their places of nature that if it was destroyed then the nymph bound to that place died with it. They were also capable of consorting with human men and women, and could produce children with them.
At the birth of Narnia, one female nymph was seen with two fauns, a dwarf, a bird and a hare, as they welcomed Digory Kirke and Polly Plummer. Nymphs were also later seen at the coronation of the King Frank and Queen Helen.
Centuries later, during the Age of Winter, many Nymphs were stone prisoners of the White Witch in her courtyard, along with many other statues. During the Winter Revolution, nymphs were among the many Narnians in Aslan's camp, as they welcomed the Pevensies and the Beavers just as they were going to see Aslan. It is unknown for certain if nymphs were present in the First Battle of Beruna, but they were later seen at the coronation of the Pevensies, as they became the Kings and Queens of Narnia, so it is more than likely.