In Tolkien's work, a clan of elves skilled in mining and smithywork, who went west (along with the Vanyar and Teleri) in the early days of the First Age, and settled in a city called Tirion in Eldamar. They were close to the Vanya Aule. However, many of the Noldor returned to Middle-Earth after strife in Valinor caused partly by the machinations of Morgoth. They are also known as deep-elves and (in Tolkien's early work) gnomes.
The original king of the Noldor, Finwe, was slain by Morgoth while still in Valinor, but not until his three sons had begun to fight. The eldest son, Feanor, was the maker of beautiful and powerful jewels called Silmarils, and he was jealous towards his brothers, who he feared wished to usurp his position as Finwe's heir. When Morgoth stole the Silmarils, Feanor, who grew suspicious of the Valar, led a large section of the Noldor in pursuit, and was joined by Fingolfin and his sons. Some of the Noldor, under the third son Finarfin, remained in Eldamar.
The Noldor were put under a curse by the Valar after slaying some of their kin, the Teleri, while trying to steal boats to sail to Middle-Earth. Nevertheless, they became the major power in Beleriand for most of the First Age, and the growth and battles of their realms are the main focus of the Silmarillion. This history was, however, one of feuding and dispair.
Of the elves appearing in other works, only Galadriel is of the Noldor.
The kings of the Noldor in Middle-Earth included the sons of Feanor (Maedhros, Maglor, Curufin, Celegorm, Caranthir, Amrod and Amras); Finrod Felagund, son of Fingolfin; and Turgon, son of Finarfin. Galadriel, sister of Finrod, later became ruler of Lorien.
Relations between the Noldor and other peoples such as the Sindar are portrayed in the Silmarillion as rather fraught.