All the school books say energy is the ability to do work. But what does that mean? Well it sort of means the ability to make something happen (the nearly-well-known Dave Watson definition). Everytime a force is exerted on something through a distance (which is loosely the definition of work) something has to move, which means something happened. Not very precise, I grant you. Still, it sort of makes sense, doesn't it?

But what about heat flow? Energy can be transferred through heat flow, like when you put a pot of water on the stove and the water gets hotter. Something happened for sure. Something changed. The water got hot and eventually, if left on the hot stove long enough, will start to boil. What forces are involved in this case? There doesn't appear to be anything being pushed through a distance. Does there?

Back to the Defining Characteristics:-
For the purpose of explaining energy to beginners, I still think the best description is the following nearly-well-known Dave Watson definition:
Energy is a property or characteristic (or trait or aspect?) of matter that makes things happen, and/or, in the case of stored or potential energy, has the "potential" to make things happen.

By "happen", we mean to make things move or change condition. Examples of changes in condition are changes in shape, volume, and chemical composition (results of a chemical reaction). There are also changes in pressure, temperature, and density which we call a "change of state" in thermodynamics. Phase changes, such as changing from solid to liquid, or liquid to vapor, or back the other way, are also good examples of condition changes. You know, something happened. Without energy nothing would ever change so nothing would happen. You might say energy is the ultimate agent of change.

Whenever anything happens or changes there is an energy change. Either energy changes form, as when a generator changes mechanical energy into electrical energy; or energy changes location, as when heat flowing too fast out of your body makes you cold, or heat flowing into a pot of water makes the water turn into steam. You get the idea.

Energy is a measure of being able to do mechanical work. This is a fundamental concept pertaining to the ability for action. In physics, it is a quantity that every physical system possesses. This quantity is not absolute but relative to a state of the system known as its reference state or reference level. The energy of a physical system is defined as the amount of mechanical work that the system can produce if it changes its state to its reference state; for example if a liter of water cools down to 0°C or if a car hits a tree and decelerates from 120 km/h to 0 km/h.

Energy of an object can be in several forms, potential—due to the position of the object relative to other objects; kinetic—energy because of its motion; chemical—due to chemical bonds between atoms that make up the substance; electrical—due to its charge; thermal—due to its heat; and nuclear—due to the instability of the nuclei of its atoms. In the case where the "object" is an electromagnetic wave or light, then radiant energy can also be defined.

One form of energy can be readily transformed into another; for instance, a battery converts chemical energy into electrical energy, which can be converted into thermal energy. Similarly, potential energy is converted into kinetic energy of moving water and turbine in a dam, which in turn transforms into electric energy by generator. The law of conservation of energy states that in a closed system the total amount of energy, corresponding to the sum of a system's constituent energy components, remains constant. This law follows from translational symmetry of time (that is, independence of any physical process on the moment it started). Some works (thus some forms of energy) are not easily measured by the unaided observer.