What is the difference between internal energy, temperature, and heat?

Internal energy, temperature, and heat are closely related but distinct concepts. The microscopic particles that make up a body are in constant motion and may exert forces on each other. As such, they each possess some kinetic and potential energy, which summed together give the body's internal energy. Because the internal energy of a body increases the more particles it has, we define the temperature of a body as the mean internal energy of its particles, which means that our measure how hot or cold a body is does not depend on its size. Finally, heat is the energy transferred between two bodies due to their temperature difference, and it always flows from hot to cold. Bodies themselves do not have "heat", only internal energy; it only makes sense to use "heat" when energy transfer occurs between two bodies due to a temperature difference.