Matter (or mass) is the physical stuff of the universe, while energy is what causes matter to move and change. There are different forms of matter (for example, solid, liquid and gas) as well as different forms of energy (such as electrical, chemical and nuclear energy). For centuries, scientists thought that matter could not be created or destroyed--it could only change form. The same idea seemed to apply to energy.
Rethinking the Laws of Nature
Einstein's work on the Special Theory of Relativity prompted him to rethink the fundamental laws of physics. He realized that one of the long-held views of nature--that matter could not be created or destroyed--was wrong. Einstein showed instead that matter can be destroyed and converted to energy. Conversely, energy can be converted to mass.
Einstein's equation E=mc2 demonstrates the unexpected finding that energy and mass are interrelated: Mass is a form of energy, and energy is a form of mass. The equation also helps explain the energy source for a variety of physical phenomena, from stars to the atomic bomb--although initially Einstein did not anticipate any practical applications for his formula.