When quantum physics is used to describe physical events, the descriptions have a kind of fuzziness. For example, if one tries to pin down the position of an electron, then the electron's velocity necessarily becomes less precisely known -- the Uncertainty Principle. This fuzziness is not just a product of our ignorance, but seems to be built right into the fabric of reality. In the quantum world, precise description and prediction is impossible. The best we can do is calculate the probability that when we make an observation a certain thing will be observed.
Erwin Schrodinger, one of the founders of quantum mechanics, imagined a cat inside a box. Also in the box is a little bit of radioactive material, and a radiation detector that will trigger a trip on a hammer poised above a glass vial containing cyanide. If even a single particle is emitted by the radioactive substance, the hammer will fall, the vial will break, and the cat will die.
With the box closed, let us turn on the detector for just long enough that the probability of a particle being emitted is 50-50. So what is in the box after the experiment is run? A dead cat? A live cat? Apparently, there is no way to know without opening the box and looking in.
But maybe, just maybe, nature doesn't choose between two equally probable outcomes. Maybe both outcomes occur. The particle is emitted and it isn't emitted. The detector trips the hammer and the detector does not trip the hammer. Instead of settling for one or the other outcome, the universe splits into two simultaneous, non-interacting universes. In one universe the cat dies; in the other universe the cat lives.
This is the so-called "many worlds" interpretation of quantum mechanics, and if it's true, then even at this instant your universe is splitting into a myriad of parallel universes, in each one of which your life unfolds in different ways. It is an unsettling concept, but one that I have become used to.
As longtime visitors to his site will know, I live in three parallel universes: a New England town, a fishing village in the west of Ireland, and a quiet little island in the Bahamas. Different food, different dress, different activities, different habits, different friends. The worlds are essentially non-interacting, like the branching worlds of quantum mechanics. It is all I can do to keep three worlds straight. Tomorrow morning I will inhabit another universe. Will the cat be alive or dead? We'll have to open the box and see.