Energy in physics is defined as the ability to do work. That makes sense, right? After all, the more energy you have, the more schoolwork you can do...right. So anyway, another way to see it is that work is the force exerted multiplied by the distance, or W=FD. That also makes sense. If you push a really heavy object a certain distance, you do more work than if you push a lighter object that same distance.
The energy stored in the various sources covered in this page is, in one way or another, derived from the energy of the sun. For instance, when we burn wood, we are simply converting the sun's power that was stored in the growing plant into radiant (light) and thermal (heat) energy. Hydroelectric power is derived from the running water that flows from higher elevations to lower elevations. Again, it is the sun that provided the energy to evaporate water from lower heights and transported it to the upper heights as rain. So all the energy we deal with comes from the sun.
You've already read about kinetic and potential energy. Other types of energy that are discussed in this site are derivatives of kinetic energy. For instance, thermal energy is kinetic energy in the form of heat.
Read on to discover a more in-depth view of the physics of energy.