Energy comes in six forms:  chemical energy, electrical energy, radiant energy, mechanical energy, nuclear energy and thermal energy.  These six forms of energy are all related.  Each form can be converted or changed into the other forms.  For example, when wood burns, its chemical energy changes into thermal (heat) energy and radiant (light) energy.
 
Not all energy conversions are as simple as burning wood.  An automobile engine is a complex tool that converts the chemical energy in a fuel into mechanical energy, the energy of motion.

Figure 4 Forms of energy Any of these forms of energy can be changed into the other forms.