Chaos theory states that things are not really random, just complex. Many apparently random events can be represented by a simple computation which, when iterated, produce complex results. That is a formula which is constantly repeated can produce models of trees and weather system. The values from each stage are feed back into the next stage, also known as feed back. The result of iteration, formulas and feedback is a chaotic system.
Most natural processes are chaotic systems as well as many man made ones.
The result is a complex system which although possible to predict generally is hard to predict in detail. Any one who has, at one time or the other, played at dodging the wave at the beach will know that. A man made example of a chaotic system is the financial markets of the world. These, from the outside, appear to be random and unpredictable, but, as we know, each financial decision is a conscious decision and is far from random. Each step of the way computations are used, along with previous knowledge to make these decisions.