In this series, Life's Little Mysteries explains complex subjects in exactly 200 words.
Quantum mechanics is the body of laws that describe the (bizarre) behavior of the fundamental particles, such as electrons and photons. First, these don't behave like tiny balls, but rather like fuzzy clouds that are spread over a large area. Each particle is described by a "wavefunction," or probability distribution, which tells what its properties (such as its location and velocity) are more likely to be, but not what they are. Those properties aren't discreet; a particle is distributed throughout space like a wave, and it has a range of velocities. But when you experimentally measure its location, for example, its wavefunction "collapses" and it appears in only one spot. However, there's no way to simultaneously measure both a particle's location and its velocity. The more precisely you know one, the fuzzier the other one is. Called the "uncertainty principle," this isn't merely a limitation of our measurement capabilities, but rather an inherent fuzziness in the state of the universe. Also, no two electrons (among other particles) can have exactly the same wavefunction. This forces electrons to occupy different energy levels around the nuclei of atoms, and it causes matter — e.g. stars, you — to be stable and distributed throughout space.