The energy used by an organism is called ATP. Part of its makeup is three phosphates. When one of these breaks off, energy is released and ADP is formed.
ATP stands for Adenosine Triphosphate. It is made up of an adenosine molecule and three inorganic phosphates, or a triphosphate. When one of these phosphates is removed, the energy that keeps human beings alive is produced. Therefore it is a crucial reaction in order to sustain life.
When one of the three phosphates are removed the resulting compound is called ADP, Adenosine Diphosphate.
ADP can be converted back into ATP so that it can be used again. Energy is required to do this, but the but there is an overall gain in energy when the process occurs.
ATP is constantly being used by the body, so it needs to be replaced on a regular basis. This is done with glucose. So, when something is eaten and goes into the digestive system, the glucose creates ATP which can be used by the body.
The other way that ATP is created is through respiration. This applies in both animals (including humans) and plants. The difference between the two is that plants do not obtain ATP through glucose.