Complexity Theory is the study of systems that organize themselves in interesting ways.
Chaotic systems are extremely unpredictable and have no memory of the past.
Complex systems are on the border between order and chaos. They have enough order to have a memory of the past and to be somewhat predictable, but have many different possible behaviors. Complex systems exhibit self-organization, which means that the behavior of the whole system can be far more elaborate than any of its parts. Complex systems can adapt to their environment and evolve. Life is a complex system composed of organic molecules.