In philosophy, systems theory, and science, emergence is the way complex systems and patterns arise out of multiplicity of relatively simple interactions. Emergence is central to the theories of integrative levels and of complex systems.
Early examples of emergent behavior came from something called the game of life. Where very complex patterns were observed despite there being just a few rules governing how the system worked. Scientific American article on Life from the 1970's. Straitforward description of the cellular automation.
When games get their intelligence from emergent behavior, that behavior tends to be much more robust and adaptable than when you try to program AI at a high level. When behavior is emergent then you can tend to just program in goal states at a high level and let the individual units adapt to try to maximize for the new goals.
This is a list of articles on emergent behavior and AI:
How the AI in AI War: Fleet Command work. This is a very fascinating way to create intelligence in a game.