Internal Energy is the energy stored in a system at the molecular Level. The System's Thermal Energy -the Kinetic Energy of the atoms due to their random motion relative to the Center of Mass plus the binding energy (Potential Energy) that holds the atoms together in terms of atomic bonds.
We consider all possible internal changes to the body as making up the total internal energy.
There are two ways to change the internal energy: with work, and everything else. Everything else is defined as heat. Heat is the defined as the transfer of energy to a body that does not involve work or those transfers of energy that occur only because of a difference in temperature. As Bellman would say,
Repetition being the apotheoses of pedagogy therefore, heat is the workless transfer of energy.