Energy types

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When anything happens around us it involves energy being changed from one type to another. This could be you talking, the electricity going into your computer to light up the screen, a log burning on a fire or a weight bouncing up and down on a spring.
Its important to remember the law of conservation of energy.  This is that we can not make energy, or destroy it, but only change it from one type to another.  The key types of energy you need to know are:
Gravitational Potential energy, (gained when lifting object up, for example being lifted on a ski lift)
Elastic potential energy, (stored in things that are stretched, for example in a catupult which has been stretched)
Chemical energy, (stored in materials and released in chemical reactions for example in your food, magnesium about to be burnt or an electrical cell)
Kinetic energy,(the energy of moving things, like a moving car)
Electrical energy  (the energy carried by a current, through the wires in your computer)
Thermal energy (heat energy nearly anywhere!)
Sound energy (the energy released by things vibrating, like a guitar string)
Light energy (the energy carried in electromagnetic waves some of which we can see, from example from a torch or the Ultra Violet rays that burn us when we forget sun cream.)
The first three are all potential energies.  They can be stored and have the potential to be released at any time.
So an battery powered torch stores chemical energy in the cell.  This is moved as electrical energy as the current flows through the wires and is released as heat and light from the bulb (see this video).  So in the torch the light is the useful energy and the heat is wasted.
Or in a fire the chemical energy stored in a log is again released as heat and light, as happens when any fuel is burnt.
The amount of energy involved in these changes can be estimated quite easily.  If a fly crashes into you the feeling is very different if a rugby tackle is made on you at the same speed.  So the mass of the object makes a big difference.  If the rugby tackle happens slowly it is very different to one at full speed, and so on.  We measure the amount of energy in Joules (but of course still use calories as well when we think about the energy in food).