This document defines key concepts in thermodynamics including energy, heat, work, and their relationships. It makes the following key points: 1) Energy exists in many forms and can be stored within or transferred between systems. The stored forms include kinetic, potential, and internal energy, while the transient forms that can be transferred are heat and work. 2) Work is done when a force causes a displacement, and in thermodynamics is defined as the sole effect of raising a weight. Heat is transferred between systems due to a temperature difference. 3) Both heat and work are transient phenomena that cross system boundaries during state changes, but they differ in that work involves organized energy transfer while heat is less organized