The Norns of Norse mythology are three old crones by the names of Urd (that which has become), Verdandi (that which is becoming) and Skuld (should).
They live beneath the roots of Yggdrasil, the world tree at the center of the cosmos (although some accounts have it that they dwell above the arch of the Bifrost Bridge), where they weave the tapestry of fates. Each person's life is a string in their loom, and the length of the string is the length of the person's life. Interestingly, Skuld was also the name of a Valkyrie.
Thus everything is preordained in the Norse Religion: even the gods have their own threads, though the norns do not let the gods see those. This clear subjection of the gods to a power outside their control and the implication that they, too, will have an End are major themes of the literature surrounding norse mythology.
The three weaving crones who control destiny exist at a deep mythic level, though probably not as old as the art of weaving itself. The counterparts of the Norns among the Greeks were the Moirae, known to the Romans as the Parcae.
In Macbeth by William Shakespeare, three sisters called the Weird Sisters tell the protagonist about his destiny. Weird seems to be derived from the Anglo-Saxon wyrd, which is cognate to urd.
Three characters very loosely based on the Norns, and named as such, appear in the Japanese manga and anime series Oh My Goddess.
Norn was also a language. See Norn language.
Norns are also a fictional species in the computer game series Creatures. See Norns (Creatures).