What is Gibbs free energy and what does it indicate about spontaneity?

Gibbs free energy (G) is defined as G = H - TS, combining enthalpy and entropy effects. At constant temperature and pressure, the change in G determines spontaneity: dG < 0 indicates spontaneous process, dG = 0 indicates equilibrium, dG > 0 indicates non-spontaneous (requires work input). For chemical reactions: dG = dG_standard + RT*ln(Q), where Q is reaction quotient. At equilibrium, dG = 0, giving dG_standard = -RT*ln(K). Gibbs energy minimization is used to calculate chemical and phase equilibrium.