A Norn is an AIL (Artificial Intelligent Life) species, the development and survival of which constitute the main thematic elements of the program.
Norns are playful, all-trusting, naive creatures that love to explore. This is often the reason for their downfall, suffering, and death. They are especially susceptible to disease in comparison to Ettins and Grendels, and in old age they may fall prey to a wasting disease that prevents them from absorbing certain nutrients.
The Shee originally created the Norns as a mixture of valet and pet. According to the Creatures Labs website, they spread across the galaxy through the Warp and evolved into various variations, including but not restricted to: Toxic Norn, Treehugger Norn and Magma Norns. The spread of Norns throughout the galaxy is related to the mysterious disappearance of the Shee from Albia (from the point of view of the Norns). It may also be a sign of how the Shee fared in their attempt to find a habitable planet.
In the games, the Norns have their own simulated biochemistry, haploid genetics, and neural network to serve as a brain. From Creatures 2 onwards, Norns also possess organs determined by their genetics; mutation and injury can render these defunct, with typically fatal consequences. Each of these has grown somewhat over the course of the series, allowing the creatures to become more complex and intelligent. Norns are omnivores and eat a variety of foods, including carrots, lemons, seeds, insects, fish, honey, and other things. The Norns that the player starts with require some degree of training and discipline from the player in order to make good decisions to maintain their health; however, later on they can evolve to be more independent and hardy, with better-developed instincts and a more robust biochemistry. The player can take advantage of this to breed certain colors or traits through artificial selection, and through many generations the changes can become quite pronounced.