Internal energy is the total energy contained within a system, primarily due to the motion and arrangement of its particles. It includes both kinetic energy, which arises from the movement of particles, and potential energy, which is related to the forces acting between them. Changes in internal energy can occur through heat transfer or work done on or by the system. For example, when a substance is heated, its internal energy increases, leading to changes in temperature or phase, such as melting or boiling. Understanding internal energy is crucial in fields like thermodynamics and physical chemistry.