Geraint and Enid are the principle figures in the both Welsh tale of Geraint, the Son of Erbin, and a similar story by Chrétien de Troyes. Chrétien's Erec et Enide, written c. 1170, is his earliest surviving Arthurian romance. The Welsh story dates from the thirteenth century and is later than Chrétien's.
In both versions, the hero wins a tournament in which the knight defends the boast that his woman is the most beautiful and fights for the prize of a sparrow hawk. When Enid and Geraint (or Erec) marry, the knight is so happy that he neglects his knightly duties. Geraint, hearing his wife bemoan that she has been the cause of his shame, believes that she must have been unfaithful. Together they embark on a quest that proves Geraint to be a true knight, and her to be a faithful wife.
Chrétien's romance is the the basis of Hartmann von Aue's Erec and the thirteenth-century Icelandic Erex Saga. Tennyson used Welsh tale, Mabinogion as the basis of his story of the two characters. Called "Enid," it was one of the first four idylls published in 1859 in the volume called Idylls of the King.
Full references to more about Enid & Geraint at the Rochester site
Characters from King Arthur's legends