The three states of matter that are most often studied are gas, liquid, and solid. In the diagram is a fourth state of matter, plasma, which is much hotter than the gas it forms from.
Notice that that only plasma can only be formed when a gas is ionized (changed into charged particles).
To make a change from one state of matter requires a change in energy. For example, the change form the liquid state to a gas is called vaporization. The amount of energy (heat) needed to make this change is called the enthalpy (heat) of vaporization. Since the gas state of matter is hotter than is the liquid state, enthalpy of vaporization is the amount of heat that must be added to the liquid for it to change into a gas.