Heat transfer is thermal energy in transit due to a special temperature difference.
Whenever there exists a temperature difference in a medium or between media, heat transfer must occur.
Different types of heat transfer processes as modes. When a temperature gradient exists in a stationary medium, which may be a solid or a fluid, the term conduction is used to refer to the heat transfer that will occur across the medium, in contract, the term convection refers to heat transfer that will occur between a surface and a moving fluid when they are at different temperature. The third mode of heat transfer is termed thermal radiation. All surfaces of finite temperature emit energy in the form of electromagnetic wave. Hence in the absence of an intervening medium, there is net heat transfer by radiation between two surfaces at different temperatures.
Note: all lecturers are takes from references:
1) Fundamentals of heat and mass transfer. Incropera.
2) Heat transfer a practical approach, Yunus A. Cengel.