Plasma is the fourth state of matter. Heat up a solid and you get a liquid. Heat up a liquid and you get a gas. Heat up a gas and you strip the electrons off the gas molecules. Instead of a collection of neutral gas molecules, banging into each other, you now have a collection of charged particles -- negatively charged electrons and positively charged ions. But now every particle can talk to every other particle (without banging into each other) simply by its electromagnetic force! It's a totally new state of matter, and is much more exciting than a boring old gas. Now there can be long-range effects, collective effects (waves), wave-particle interactions, and a whole bunch of nasty non-linear physics! Although most everyday objects are not plasma (with a few exceptions: fluorescent lights, fire, lightning, the northern lights), over 99% of the known universe is in the plasma state: everything from hot stars to the cold interstellar medium.