Heat transfer describes the flow of thermal energy due to temperature differences. There are three main types of heat transfer: conduction, convection, and radiation. Conduction involves the transfer of heat through direct contact of materials without bulk motion. Convection occurs through the movement of fluids like air and water carrying heat away from heated surfaces. Radiation is heat transfer through electromagnetic waves between bodies not in contact through space. Heat transfer has many applications in fields like electronic cooling, aircraft engines, space shuttles, and cryogenic systems in aerospace engineering and space environments.