PLASMA BURNING:  

Plasma cutting is a process by which work an electric arc is sent through a gas that is passing through a constricted opening.  The gas can be shop air, nitrogen, argon, oxygen. etc.
 
The temperature of the gas is elevated by this electric arc to the point that the base material enters a plasma state.  As the metal is being cut, the electrical conductivity of the plasma causes the arc to transfer to the work.
 
The restricted opening in the nozzle allows the gas to pass through causing it to squeeze by at a high speed.  This high speed gas cuts through the molten metal.  The gas is also directed around the perimeter of the cutting area to shield the cut.