Complex systems theory is a branch of science that studies complex systems, which are systems made up of many interconnected parts that can give rise to emergent behavior. This means that the behavior of the system as a whole cannot be predicted by looking at the individual parts alone, but only by considering how they interact with each other. Examples of complex systems include the human brain, social networks, and ecosystems.

Complex behavior refers to the behavior of a complex system, which cannot be predicted by looking at the individual parts of the system alone, but only by considering how they interact with each other. Complex behavior emerges from the interactions between the parts of the system, and is often characterized by patterns, fluctuations, and self-organization. Examples of complex behavior include the formation of galaxies and the emergence of consciousness.