The flapping of a single butterfly's wing today produces a tiny change in the state of the atmosphere. Over a period of time, what the atmosphere actually does diverges from what it would have done. So, in a month's time, a tornado that would have devastated the Indonesian coast doesn't happen. Or maybe one that wasn't going to happen, does.

Does God Play Dice?: The New Mathematics of Chaos (ed. Penguin UK, 1997) - ISBN: 9780141928074