Energy dominates our lives; anytime, everywhere, in many ways. Energy shows up in a variety of energy forms, for example when an object moves as kinetic energy, when lifted as gravitational energy, when compressed as deformation energy, when heated as thermal energy, and in the energy system as electric, magnetic, chemical, or nuclear energy. Energy can be converted between energy forms while it is overall conserved. How is it possible that a single term describes and relates such a variety of different things? Moreover, despite using the term "energy" almost anytime and everywhere, there is today no satisfactory explanation of what energy is; definitions range from "the ability to do work" to being "a number." Understanding the basics of energy takes the reader on a discovery of what energy is. It is based on the common concept that nature can be understood looking at the motion and interaction of objects, like particles, and applies this concept for the first time systematically from simple to complex situations. This way, the basics of energy can be understood, like the origins of the different energy forms, how energy is converted between them, and why energy is overall conserved. The book includes many examples and the historic background how the concept of energy evolved.