*Provided by Ails*
The Tuatha Dé Danann ("People of the Goddess Danu") were one of the mythical races who settled in Ireland before the arrival of the ancestors of modern Gaels. The Dananns were descendants of the goddess Danu. Her son Dagda was their most powerful leader of the Dananns. They were also known as The Children of Morrigan as Danu was the Morrigan. It is generally accepted that Morrigan has the meaning of Great Queen or possibly Phantom, i.e. Otherworld Queen. It is certainly a title rather than a name.
The Tuatha Dé Dananns were a race of deities as well as race of heroes. They were skilled in art and science, poetry and magic.
They were said to come from four mythical cities: Falias, Gorias, Finias and Murias. When they came to live in Ireland, the Dananns received four magic treasures or talismans, one from each city. Before the Tuatha Dé Danann migrated to Ireland, they had learned all their skills from for four wizards/bards (druids) from these four cities.
After the mortals defeated the Dananns, the Dananns either retreated to Tir na n-Og ("Land of Youth") otherwise known as the Otherworld or they continued to lived on the land with the mortals, but their homes (subterranean palaces) were hidden by magic from the eyes of mortals.
In the Otherworld, the Danann remained young and seemingly immortal. Immortal in the sense, they can live a very long life and remain young, but they can be killed and destroyed, just like any mortal.
There were frequent visits of the Dananns with the mortals. Sometimes they aided mortals, while other times they seek their destruction. Sometimes they sought marriage with mortals. Most of the times, the Dananns would come to the surface and meet their lovers, other times the mortals were allowed to live with them.
In some myths, the Tuatha Dé Danann were still seen as Celtic deities. However, in others, the Dananns had degenerated into nothing more then fey people; in another words, the Dananns became the "Fairy People". The Tuatha Dé Danann became frequently associated with fairies. Because of the Christian influences in the myths, some of them died of old age when they left the Otherworld and they were baptised before their death.
(It should be noted that the fairies in Celtic myths (especially Irish, Welsh and Arthurian myths) had nothing to do with tiny pixie with wings that are found in folklore and children fairy tales, like Tinkerbell in Peter Pan or the Fairy Godmother in Cinderella. The fairies found here were human with supernatural power. Modern interpretations of fairies tend to prettify them, particularly during the Victorian period (19th century) in Britain.
In early Irish and Welsh literature, they could be tall or short, beautiful or ugly. They can be benevolent beings, but at other times they can be frighteningly cruel or malign.
These are some artists impressions of the Tuatha Dé Danann: