Plasma is a state of matter (akin to solid, liquid and gas) where the matter has become ionized, or the electrons stripped from the atoms. Therefore, a plasma consists of both negatively charged electrons, and positively charged ions.
Plasmas found on earth include fire and electrical arcs. The sun, stars and interstellar clouds consist mainly of this form of matter.
Like a gas, in a plasma the constituents are not physically constrained in position by each other, and will expand to fill the available space. However, the free ions and electrons may be controlled using electric and magnetic fields. Barring cold fusion or electrostatic confinement, sustained magnetic confinement of hot plasmas must be achieved in order to harness nuclear fusion as a power source.