1) Conduction- It is the mode of heat transfer from one part of substance to another part of same substance or one substance to another without displacement of molecules or due to the vibrations of molecules.
Example-Heat transfer in metal rod.
2) Convection: It is the mode of heat transfer from one part of substance to another part of same substance or one substance to another with displacement of molecules or due to the fluid flowing.
Example: Heat flow from boiler shell to water.
3) Radiation: It is the transfer of heat through space or matter. For Radiation there is no need of medium as like convection and conduction. It passes through vacuum in the form of electromagnetic waves.
Example: The heat energy receives from sun to the earth surface.