The concept of energy is a longstanding enigma. An acceptable explanation is given by finding a new concept of energy: effective energy which is defined as the conserved quantity transferred between two interactions, not between two systems. The effective energy is not a property of a system, but that of a composite system. It is a kind of relation, or distance, between two systems. The conservation of effective energy is composed for reversible processes of a composite system. The conventional energy and exergy (available energy) are explained as special cases of the effective energy. Energy conservation for reversible processes can be inferred from the conservation of effective energy. In irreversible phenomena effective energy irreversibly changes into entropy. It is revealed that the energy conservation in irreversible process is not conservation in general but just a special expression when the absolute temperature scale is adopted.