What is this creatures series?
Creatures was a PC game series that provided a simulated world/ecosystem for little virtual lifeforms called Norns to live in. Norns acted independently with guidance from the player, who took the form of a floating Hand that could carry/use objects, push buttons, type words to the Norns, and give punishment/reward.
Norns had 'brains' that could learn through stimulous and interaction. They could also be taught a very basic language, but would gradually understand how to construct meaningful sentences with the words they learnt in order to express their feelings, or in later games, even suggest a course of action to another Norn who had expressed his within earshot. The world they lived in contained various interactive objects like toys, food, elevators, buttons, computers and other contraptions. When encountering an unfamiliar object, curious Norns would experiment and see how they could manipulate it by trying various things: pushing it , pulling it, carrying it, trying to eat it... etc. Some Norns learnt that many things could harm them and would instinctively run away from 'scary' things.
Norns also had simulated biochemistry and genetics, and would grow up, breed, and die in a natural cycle. Offspring would be the result of combined genetic traits of their parents and could suffer mutations and adaptations that in some cases were beneficial in their survival. None of these things were scripted events within the game; they happened 'naturally'. People bred Norns that lived longer, could breathe underwater, and in some cases even became immortal and unable to die natural deaths. Norns could also colour mutate. Some people, for some ungodly reason, really loved having eye-gougingly bright neon creatures running around... Even things like their gait (pose) could mutate or be affected by ill health, causing weaker creatures to hunch over or crawl if they could not properly walk.
Creatures 1 was the purest concept, because it was Steve's brainchild. But after the game took off and their publisher demanded dumbed-down, simpler, less scientific sequels, Creatures 2 and 3 ended up being little more than graphical 'enhancements' (Personally I just think they made everything look like smoothed plastic). They did expand the Norn's behaviours and intelligence, though.
One of the best things about Creatures were the 'Kits' that players could use to peek into the inner workings of their creatures. The Genetics Kit, Medical Kit, Breeder's Kit, Science Kit etc allowed you to closely study what was happening inside a Norn's brain, or monitor its health, or fertility. There was a lot of support for people to create their own Norn breeds, genomes, objects, and even entirely new worlds. It was very much a modder's paradise, especially for people who loved tinkering with AI.
For more info:
http://creatures.wikia.com/wiki/Creatures_Wiki_Homepage