ATP

Even though the ATP molecule has been studied for several decades the  1997 Nobel prize for Chemistry has been awarded to 3 biochemists for the study of the important biological molecule, adenosine triphosphate.
ATP is the energy currency of life. It is used to power many metabolic processes.
 

ATP energetics

ATP works by losing the end most phosphate group when instructed to do so by an enzyme. This reaction releases a lot of energy, which the organism can then use to build proteins, contact muscles, etc. The reaction product is adenosine diphosphate (ADP), and the phosphate group either ends up as orthophosphate (HPO₄) or attached to another molecule (e.g. an alcohol). Even more energy can be extracted by removing a second phosphate group to produce adenosine monophosphate (AMP).

the energy is released HPO4 is produced (when bonds are broken)

The reverse reaction takes place and the phosphate group is reattached to the molecule using energy obtained from food or sunlight.
Thus the ATP molecule acts as a chemical 'battery', storing energy when it is not needed, but able to release it instantly when the organism requires it.