?These regions of strong field due to space charge which cover the electrodes will be referred to as the sheaths. The relatively field-free regions beneath the sheaths where positive and negative space charges are nearly balanced will be called the plasma.?-- Irving Langmuir  1929

A.  Thermal

B.   Magnetically Confined -- requires external initiation

C.  Discharge

A.  Natural Plasmas

       1.  Stellar Coronae - caused by intense photon bombardment, strong stellar magnetic  fields

       2.  Planetary Ionospheres - high energy photons, plantetary magnetic fields. [1]

       3.  Lightning

B.  Artificial Plasmas

       1. Thermal - rocket exhausts, high power lasers,  nuclear explosions

       2. High vacuum magnetic ... requires a "starter"*

       3. Discharge plasmas - arc welders, HVDC vacuum tubes, metal film deposition machines

*Note that a discharge starter is needed to start a plasma in an artificial  magnetic system.  UV and higher frequency photons from lasers can also ionize a rarified gas sufficiently.