energy

Energy is the ability, or potential, to do work.

It is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat and light. Energy is a conserved quantity - the law of conservation of energy states that energy can be converted in form, but not created or destroyed. That means that we can look at any closed system at some moment in time, compute the total energy in it, and assuming we have not missed any of the forms of energy, then without predicting the behavior of the system in detail, at any future or past time, the total energy will be the same as it was now. This is a fundamental observation of the universe.

This makes our definition "energy" refer to something theoretically meaningful and practically useful. This is pretty much the reason why energy is defined as such.

Source.

This view works well so long as you look at the world in the same way. But that doesn't always happen.

For example, you might watch a car while standing on the sidewalk. You conclude that it has a high speed and therefore a lot of kinetic energy. You are right. But the driver sees things differently. He sees the seat is right under him. A while later, the seat is still right under him. It hasn't moved. He concludes the car isn't moving and has no kinetic energy. The world rushing by outside is moving and has a lot of kinetic energy. He is also right.

It might sound silly to say the world is moving, when it is obviously stationary. But it isn't stationary. It is flying around the Sun. So the Sun seems to be stationary, but it is flying through the galaxy. It turns out that all of these points of view are equally valid.

You can always choose yourself as an object moving at speed 0. You can measure the speed of objects around you and calculate their energy. You are right. But the driver of a car can do the same thing. He will get different speeds and different energies. And he is also right.

This means that energy really isn't some kind of stuff. A better view is that it is like an accounting system. You calculate how much energy is present at the start. Stuff happens. Perhaps a speeding car coasts up a hill and comes to a stop. You calculate how much energy is present. You will get the same answer.

It is common to think of energy as some sort of stuff that can live inside a moving object as kinetic energy, or maybe stored in a stretched spring. We get so used to thinking in terms of energy and fields (like electric fields) and treating them as real things that we forget they are just tools to approximately describe the universe and not the universe itself. We may say the reason why that a moving object slows as it compresses a spring is that energy is conserved. Physics has progressed so much and matches the behaviour of the universe at such accuracy that we often times forget the difference.

Forms of energy include the kinetic energy of a moving object, the potential energy stored by an object (for instance due to its position in a field), the elastic energy stored in a solid object, chemical energy associated with chemical reactions, the radiant energy carried by electromagnetic radiation, the internal energy contained within a thermodynamic system, rest energy associated with an object's rest mass, etc. These are not mutually exclusive.