Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode (which is usually but not always made of sintered tungsten) and the workpiece. The key difference from GTAW is that in PAW, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities (approaching the speed of sound) and a temperature approaching 20,000 °C.

some useful links:

1. http://en.wikipedia.org/wiki/Plasma_arc_welding
2. http://www.processwelding.com/pws_new/plasma_welding.htm
3. http://www.robot-welding.com/arc_weld_processes.htm
4. http://www.answers.com/topic/plasma-arc-welding?cat=technology
5. http://www.robots4welding.com/applications.php?app=plasma+arc+welding