The Norns come from Nordic mythology and are often compared to the Fates that are part of Greek and Roman mythology. They are very similar, but differ in important ways.
The Norns are three women, one a young maiden, one a middle aged mother, and the other an old crone. Unlike some other traditions, the Norns can also be shown as three old women only, or three middle aged women. They don’t have to be shown just as the maiden/mother/crone trio. Norns are usually shown sitting in a circle, sometimes standing, and holding scissors and thread. Sometimes the thread is replaced by scrolls, as the Greek/Roman version of the Fates cutting a thread for each individual doesn’t translate into the Nordic tradition. Sometimes one of the Norns is shown holding a set of scales, as if to weigh the life of a man before it’s cut by her sister, but the scales aren’t actually one of the attributes of the Norns and are something of an artistic license.
There are no actual myths of the Norns. Instead they are mentioned in passing by characters in other myths, and described in Snorri Sturluson’s Edda. There are actually many Norns, of many different kinds, but these particular three are now taken as representative of all the Norns to those who don’t study Norse mythology or culture very deeply. That is where the Norns differ from the Fates. The Fates are three and three only. The Norns are many and only these three sisters are important.
The three sisters were Skuld, Urd, and Verdandi. Skuld is the sister of the Future, Verdandi is the sister of the Present, and Urd is the sister of the Past. Those translations aren’t entirely correct, as the words for each goddess are actually forms of the Old Norse words “vertha” (to happen) and “skulu” (to must, to have to), and come to mean “happened,” “happening,” and “to must.” Another way to translate Skuld’s name, since the word for her name is actually in the past tense but she’s the goddess of the Future is: “has to will have happened,” and her name translated this way feeds very much into the Norse idea that there is no escaping one’s doom. (Sorry if the linguistics aren’t very interesting to anyone else–I’m a Linguistics major and I found it fascinating!)
A Norn seen crying was the ultimate sign of Doom, which played a major part in Norse mythology and thus their culture.
On the up side, there’s a positive to knowing what’s going to happen. The Norns, unlike the Fates in some of their stories, aren’t malicious in deciding a person’s fate. They’re more impartial in deciding the fates of gods, men, dwarfs, and giants.
There’s also a down side to knowing the fate of someone. They could have changed someone’s fate if they so chose, if they felt it was too bleak. However, this would imply that the Norns had emotions and feelings about a person’s depressing fate and wanted to change it.
Honestly, I’m not sure how to describe the Light and Dark sides of the Norns. There are no myths about them that would speak to their personalities. Mostly the Norns are an idealized generalization of three women who decide a person’s destiny. They are the personification of a person’s life and of Time. What are the Light and Dark sides of Time?