In addition to Kinetic Energy or "Energy of Motion", there are other forms of energy. All these other forms may be classified as "Potential Energy", PE. We will be interested mainly inGravitational Potential Energy, PEg
Elastic Potential Energy, PEel
Other forms of energy include nuclear energy and chemical energy.
Heat or thermal energy is a form of kinetic energy on a microscopic level.
I will usually write potential energy as PE but it is also common to write it U (eg, The Mechanical Universe video tapes will always use U for potential energy).
(c) 2005, Doug Davis; all rights reserved