Heat is related to thermal energy as the process of transferring thermal energy from one object or substance to another, according to UC Davis. The amount of thermal energy in an object or substance is directly proportional to its temperature, while heat is responsible for many changes in temperature.
Thermal energy is actually a type of kinetic energy, UC Davis says. It is the energy within the random motion of the constituent particles of an object or substance. The amount of thermal energy in an object depends both on its temperature and the number of particles in an object. A small hot tub might contain less thermal energy than a nearby large swimming pool, even though its temperature is higher, since there are many fewer water molecules in motion in the hot tub.
Thermal energy in any object or substance is transferred as heat by one of three phenomena, according to UC Davis. Conduction is the direct transfer of thermal energy between particles in contact with one another. Convection is the movement of thermal energy via movement of the substance that contains it, and as such generally only occurs in fluids. Radiation is a transfer of thermal energy as types of light energy through space. Sunlight is an example of radiation.