if i could go back in time and know what i know now about energy. i would say that Gravitational potential energy is energy an object possesses because of its position in a gravitational field. The most common use of gravitational potential energy is for an object near the surface of the Earth where the gravitational acceleration can be assumed to be constant at about 9.8 m/s2. also i would say that Kinetic energy is energy of motion. The kinetic energy of an object is the energy it possesses because of its motion. The kinetic energy* of a point mass m is given by

Kinetic energy is an expression of the fact that a moving object can do work on anything it hits

and for internal energy: U is the most common symbol and internal energy happens when there is a different change of temperature