Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process. Changes at or just beneath Earth's surface due to weathering and/or diagenesis are not classified as metamorphism.
Three types of metamorphism exist: contact, dynamic and regional. Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.
Read more about Metamorphism: Limits of Metamorphism, Classification of Metamorphic Rocks, Prograde and Retrograde Metamorphism