Atoms are normally in a neutral state: the positive charge of the nucleus is equal to the negative charge of the electrons.

The agitation of a gas increases when it absorbs heat. If enough heat is absorbed,  electrons are lost from the atoms and matter is ionized, and we say that it is in the plasma state. 

All incandescent gases formed of atoms (sometimes molecules) converted into positive ions and negative electrons, and in a continual state of agitation are called plasma. The gas may also contain some atoms and molecules which have not been ionized (neutral particles).

Some example of plasmas are:

Some areas of flames, the gas in fluorescent lamps, the air in the path of a lightening bolt. The matter which forms the stars is also in the plasma state.