The loosest definition of a plasma is that it is an electrically conducting gas. At normal temperatures and pressures gases are usually very good electrical insulators. This is because the electrons in the gas are tightly bound inside gas atoms and are not free to move in response to externally applied electric or magnetic fields. Under certain conditions, however, some or all of the electrons can be removed from their parent atoms, a process called ionization. The gas then consists of a mixture of negatively charged electrons, positively charged atoms, called ions, and un-ionized neutrally charged atoms. Now the electrons and ions are free to move under the action of applied electromagnetic fields and the gas can conduct electricity.
Due to their much smaller mass, the electrons respond to the applied fields much more readily than the ions and, consequently, carry most of the current. Since electrons and ions are produced in pairs and have opposite charges most of the plasma remains electrically neutral.
There are three principal methods for ionizing a gas. The first, called field ionization, involves applying an extremely high electrical field that acts on the electrons in a neutral atom and essentially disrupts the atom. The second, called thermal ionization, involves raising the temperature of the gas until collisions knock electrons out of the atoms. Thus, a plasma does not have to be "hot", although some are extremely so. The third method involves bombarding the gas with high energy radiation or other sub-atomic particles.
Because the properties of a plasma are so very different from those of a neutral gas the plasma state is sometimes called " the fourth state of matter ". In practice the plasma state covers an extremely large range of temperature and pressure, from the gas in the fluorescent lamps in your house to the fusion reactions in the center of the sun. Although you may have to search for a plasma in your daily life, most of the visible matter in the universe is in the plasma state.
Technological applications of plasmas include: fluorescent lights, welding arcs, steelmaking furnaces, experimental fusion reactors, semiconductor processing, flat panel displays, photovoltaics, solar coatings, architectural coatings, and hazardous waste processing. - J. McKelliget 1998
Sci.Physics.Plasma Archive Home Page
"The Plasma State" an essay commisioned by the American Physical Society Division of Plasma Physics http://w3fusion.ph.utexas.edu/aps/plasmaState/PPSPage1.html
Plasma science & technology homepage http://www.plasmas.org/
Extensive additional plasma material can be found at
http://plasma-gate.weizmann.ac.il/PlasmaI.html
http://personal.lig.bellsouth.net/lig/i/n/infonet/Plasma.htm
Listings of upcoming plasma conferences can be found at the following sites
http://hibp7.ecse.rpi.edu/~connor/ieee/plasma_meetings