A plasma is a state of matter like a gas, but one in which the atoms of the gas are ionized. This means the atoms have lost some of their electrons, so the atoms in the plasma carry a positive charge. By definition, a plasma carries a net neutral charge, so the lost electrons with their negative charge move freely in the plasma.
Plasmas conduct electricity and so are useful in a number of common applications. Plasma TV displays consist of cells filled with ionized particles. Neon signs are another example of a plasma. In astrophysics, stars are composed of plasmas. Stars shine by nuclear fusion, fusing hydrogen under great heat and pressure to release energy. The heat in a star strips the hydrogen and helium atoms of their electrons, and thus stars are plasmas.