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.
Plasmas are fluid, and are therefore well described by fluid mechanics.
(??should these broad fields be broken down here or in seperate links - and if in seperate links, ho so because the naming is broad and fairly arbitrary??)