Chaos theory, you know the one where someone says a butterfly flaps its wings in China and a Hurricane happens somewhere else?
Well I'll Explain it now:
Some scientist (whose name I don't remember) was working on one of the first weather simulations. He wanted to stop it, leave, come back, and restart it. So he wrote down all of the values of all the parameters in the simulation. He came back and re-entered all of the values. The simulation acted very differently than before. At first he was confused. Then he realized that he had rounded off the numbers. The difference between the acutally number, and the numbers he used was small, so small that it was compared to the effect of a butterfly flapping its wings and changing the air pressure.
Weather, and other systems, depend very heavily on current conditions, and small differences in current conditions will grow to huge differences over time. This is one reason why you should forgive your weather man if he is wrong about next weekend.
You may think that if we could just measure the conditions exactly we could predict the weather perfectly. If you think that, you are wrong. Heisenberg's uncertainty principle tells us that there is a limit to how exactly things can be measured. And even if we measured to the fundamental physical limit described by Heisenberg, those small errors would grow, and the weather predictions would only be accurate for a month or so.
So anyway, I basically said:
In some systems, like weather, small differences at one time grow to huge differences at a later time, and some people like to call this Chaos Theory.