Plasma physics is the field of physics which studies the dynamic behaviour of plasmas.
Briefly, it is the study of the statistical properties of a field of charged particles, called a plasma. Sometimes called "the fourth state of matter" (besides solid, liquid, and gas), plasma in this context refers to a gas that has been subjected to enough energy to dissociate atoms from their electrons (ionization), producing a cloud of ions and electrons. Because these particles are ionized (charged), the gas behaves in a different fashion than neutral gas in, for instance, the presence of electromagnetic fields.
A common fluid treatment of plasmas comes from a combination of the Navier Stokes Equations of fluid dynamics and Maxwell's equations of electromagnetism. The resulting set of equations, with appropriate approximations, is called Magnetohydrodynamics (or MHD for short).
Fields of active research include (but of course are not limited to):