Very simply, plasma is the term given to a gas that has become ionised. (An ionised gas is one where the atoms of the gas have lost one, or more, electrons and has become electrically charged.) Gases usually only become ionised when heated to very high temperatures (>5000C°), so plasma is usually a very hot substance.

Plasma is formed by passing an electrical discharge through a gas. Under normal circumstances gases will not conduct electricity, but when a very high voltage is applied the insulating properties begin to break down. As electricity starts to flow through the gas it heats up and it begins to conduct more. Eventually, it becomes so hot a plasma is formed.

Well known examples of plasmas are the Sun and lightning. However, these are examples of natural and uncontrolled plasmas. Man made controlled plasmas have been used in industry for many years and for a number of diverse applications, such as chemical analysis and the cutting of metals.

Lightning is a naturally occuring form of plasma

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