h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Dark Matter - Only about 3% of the universe is detctable to human instruments. Dark Matter is the stuff we can't see but which must be there for the universe to work.
String Theory - the theory that all matter is in factan illusion created by the vibration of quantum 'superstrings'. Requires extra dimensions to work - currently it appears that, if string theory is correct, the universe has 11 dimensions.
Quantum mechanics - the study of how subatomic particles work. Began with the discovery that light can act as both a wave and a particle. Also shows that all fundamental particles are capable of existing in more than one place at any one time.
God Particle- Also known as the Higgs Boson, this particle gives all massive particles, or fermions, their mass. It is believed that there is a 'Higgs field' throughout the universe and mass would be the effect of distortions of this field. For example massless particles, or bosons, such as photons of light would move through space without any Higgs particles attaching themselves, whereas a fermion, like an electron, would draw Higgs Particles toward it.

Thank you very much indeed for these simple definitons Mikey - very helpful.