Thermodynamics

Heat is energy in transit due to a difference in temperature.

Internal energy is a measure of the kinetic energy of the constituent particles and the sum of all potential energies of interaction among these particles.

Say the system gains energy. Some of this might be used to increase the internal energy of the system and some might be used by the system to do work.

For instance when heat is supplied to popcorn in a vessel with a lid, some of the energy is being used to increase the temperature of popcorn and some energy is used by the pop corn to lift the lid off the vessel.

The increase in temperature of popcorn signifies an increase in its internal energy.