Brightstorm explains that thermal energy transfer involves the internal transfer of energy, of which there are three major types: conduction, radiation and convection. When there is a temperature difference between the involved systems, heat transfers from the hotter system to the cooler system, which is why hot drinks become cold.
Conduction involves the transfer of heat between atoms that are in direct contact. If the atom is a good conductor of heat, the heat transfer takes place more rapidly. When a substance undergoes heating, its molecules vibrate more and gain more heat. The BBC defines convection as the transfer of heat from a hot place to a cold place, mostly in liquids or gases. It takes place when warmer parts of a liquid rise to cooler areas of the same. The particles with more thermal energy then rise up to take the place of particles with less thermal energy.
In sharp contrast, radiation does not involve direct contact between the source of heat and the object being heated. Brightstorm defines radiation as the movement of electromagnetic waves. In this type of thermal energy transfer, heat can be transferred through empty space, such as when the sun heats up the Earth.