The Uncertainty Principle, which says that more than one aspect of a particle cannot be measured simultaneously, illustrates one of several major differences between quantum physics and classical physics. This idea, first presented by Heisenberg, takes into account that a miniscule bit of material can be either a particle or a wave, depending on the circumstance.
Actually, it is neither, until someone looks at it or an experiment forces it to pick sides. This means that a number of qualities aren't defined. If a scientist measures the speed of a particle, for instance, he can't measure position very accurately; it's as though quantifying one aspect puts the other aspects more out of focus. Physicists know this and try to compensate for it in their experiments. Still, the word "uncertainty" is there for a reason. Some physicists say this is not a principle at all and instead prefer to call the concept "uncertainty relations".