Heat flows from higher temperature to lower temperature. Though it looks simple, heat transfer is a quite complex phenomenon. There are three basic modes of heat transfer.
Conduction takes place at a microscopic level. Atoms or molecules at higher temperature have high levels of energy. Through vibration, this energy is passed on to neighboring atoms and molecules. In other words, in conductive mode of heat transfer, vibrating atoms and molecules a part of their energy.
This kind of heat transfer can take place between two or more substances or through the substance. Conduction can also take place when electrons move from one atom to another. Transient conduction takes place when temperature within an object changes a the function of time.
Convection is a mode of heat transfer which takes place through the movement of collective masses of heated atoms and molecules. Convection requires actual flow of material particles whereas in conduction, the heat is transferred through vibration without the atoms or molecules leaving their original position. In convection, heat transfer takes place through both diffusion and advection.
As convection requires the actual movement of the heated atoms/ molecules, it requires presence of a fluid for heat transfer.
Radiation is a mode of heat transfer which takes place through vacuum and hence, does not need a physical medium. Radiation takes place either through vacuum or through a transparent medium. In radiative mode, heat transfer takes place through photons present in the electromagnetic waves. The random movement of atoms and molecules in heated substances results in emission of electromagnetic waves which carry the heat to be transferred.
The radiative heat transfer is governed by Stephen- Boltzman law. A body radiates heat at all temperatures above the absolute zero, irrespective of the ambient temperature.