Run Lola Run is a great movie indeed.
One idea of the butterfly effect and of chaotic systems in general, as has been explained to some degree already, is that any change in the starting conditions of a chaotic system will lead, eventually, to completely different conditions. Chaotic systems can be quite simple, one seemingly predictable system is a
double pendulum. (Check out the animated gif on the right had side of the page to view the chaotic behavior.)
Another big idea is that aside from a sensitivity to initial values of the variables in the system, chaotic systems are not periodic. If I change the length of a simple pendulum by a small amount, for example, the period of that pendulum changes and the particulars of the pendulum's behavior will be completely different from that of the original. However, both pendulums will have very similar, highly predictable and periodic behaviors that are closely related. In a double pendulum, this is not the case. If I put a graph of the positions of the two pendulums next to each other, the connection between them would not be apparent. There are other ways of analyzing a system like this to try to find connections, but they would not necessarily exist at all.
The butterfly effect is definitely chosen for its "poetic illustration" but it does appear to be real. The problem is that there is no way to predict the way in which a particular flap will actually affect the rest of the system. Heck, if I choose to repave my driveway with asphalt as opposed to using limestone gravel, or if I fart now or hold it for a minute, it will eventually change Earth's weather.
A simple mathematical example is
x
n+1 = r (x
n - x
n2)
For 0 < x < 1 and 0 < r < 4 this system is a good example of chaotic behavior. If you play with the values, for small r the values are periodic, but as r approaches a particular value, the system appears to oscillate more and more rapidly, until eventually it becomes apparently random. I made a simple excel spreadsheet with a graph to demonstrate this system.
View attachment simple chaotic system.zip