The most common form is Gibbs free energy, written as G. It combines two other quantities, enthalpy and entropy. The equation is delta G equals delta H minus T times delta S. Here, T is the absolute temperature. Enthalpy tracks heat changes, and entropy tracks disorder or randomness.
A negative delta G means the process is spontaneous and will happen on its own. A positive delta G means the process is not spontaneous under those conditions. A delta G of exactly zero marks equilibrium, where forward and reverse processes balance out.
Free energy is useful because it weighs heat change and disorder change together in one number. This matters when the two effects pull in opposite directions, since heat change alone cannot predict spontaneity. A related quantity, Helmholtz free energy, applies instead to processes held at constant volume rather than constant pressure.
Living cells constantly use free energy changes to power reactions that would not happen on their own. A reaction can be entropy driven, enthalpy driven, or a mix of both. This depends on the sign and size of each term.
Free energy predicts whether a reaction is favorable, but it says nothing about how fast that reaction will actually happen.