If two objects are placed in contact (so that energy can flow between them easily) energy will flow from the hotter to the cooler object. Energy flowing from a region of higher temperature to a region of lower temperature is called Thermal Energy. When two objects are at the same temperature, there will be no transfer of thermal energy, and we say they are in Thermal Equilibrium.
'Thermal' energy and 'Heat' energy mean the same thing. 'Internal' energy and 'Thermal' energy do not mean the same thing, but they are related. If you add Thermal Energy to a substance its Internal Energy will increase.
Internal Energy is defined as the sum of the random distribution of the kinetic and potential energies of the molecules/atoms in a substance. In order to increase either the kinetic energy, or potential energy of the molecules/atoms in a substance, you need to add Thermal Energy to the substance.
Saying that a body possesses heat energy is as absurd as saying it possesses work! that is work and heat are merely names given for energy transfers. when you heat an object and say heat energy is transferred, we mean that the molecules in the heated object are becoming more jiggly(vibrations) that is their kinetic energy is increasing. that is what is meant by saying that heat energy increases internal energy. internal energy on the other hand is a macroscopic property of the system that arises solely from interactions between the particles that make up the system. feel free to correct me if i am wrong and hope this helped