Firstly , when a temperature difference occurs between two bodies or a body with its surroundings , heat transfer occurs.
There are basically three modes of heat transfer :--
(1.) Conduction :-- Conduction is mode of heat transfer in which heat is transferred via direct contact.
For e.g :-- If we touch a hot plate or pot , we are going to feel the heat because we are touching it , its because of direct contact and is due to conduction.
Conduction generally occurs in solids and liquids.
In conduction heat is transferred due to the vibrations and collisions between the medium particles which occurs due to the source of heat.
(2.) Convection :-- Convection is the transfer of heat due to the actual movement of the medium particles.
Convection generally occurs in fluids ( liquids and gases ).
For e.g :-- When a liquid is heated it expands and this lowers its density.
The less dense liquid rises and its place is taken by more dense colder liquid.
This movement of liquid is responsible for the heat transfer and this is the basic principle behind convection.
(3.) Radiation :-- Radiation is the transfer of heat via indirect contact or not touching.
It is basically heat travelling through air or space.
It is also described as the flow of heat from one place to another by means of electromagnetic waves.
It is very common because it is the primary method of heat transfer like in case of fire and sun.
For e.g :-- If we sit near fire we feel warmth , it is due to radiation.