Plasma is a state of matter, it doesn't need a specific shape or form.
its usually heating a particle or atom till its ionized.
Then from ionized it has positive and negative electrons.
Then magnetic field pushes the opposite charge out.
Battle Angels while pretty old, have an extremely interesting grasp on how to use plasma that
40k can only dream of.
http://battleangel.wikia.com/wiki/Plasma_poison