What the Bleep Do We Know?!
Quantum mechanics incorporates four classes of phenomena for which classical physics cannot account:
- The quantization of certain physical properties
- Wave–particle duality
- The uncertainty principle
- Quantum entanglement
Electrons traveling around a nucleus couldn't have arbitrarily small or arbitrarily large amounts of energy, they could only have multiples of a standard "quantum" of energy. Source
Scientists interpret quantum mechanics to mean that a tiny piece of material like a photon or electron is both a particle and a wave. It can be either, depending on how one looks at it or what kind of an experiment one is doing. In fact, it might be more accurate to say that photons and electrons are neither a particle or a wave -- they're undefined up until the very moment someone looks at them or performs an experiment, thus forcing them to be either a particle or a wave. Source
The Uncertainty Principle
A theory by Werner Heisenberg called the Uncertainty Principle. It states that if a researcher wants to measure the speed and position of a particle, he can't do both very accurately. If he measures the speed carefully, then he can't measure the position nearly as well. Source
This is a clip from the What the Bleep to We Know video above explaining entanglement.