Recall that temperature is a measure of the average kinetic energy of particles in an object.
Remember that heat is the transfer of thermal energy from a region of higher temperature to a region of lower temperature.
When two objects of different temperatures are placed in thermal contact with each other, thermal energy transfer will take place between them. Temperature of the initially hotter object will decrease and temperature of the initially colder object will increase until they reach the same temperature (provided there is no change in state). We say they have reached thermal equilibrium.
There are 3 heat transfer processes: Conduction, Convection and Radiation which we will be exploring in this section.