ADP or Adenosine-di-Phosphate is an organic molecule used in the energy maintaining metabolism in the body. ADP is convertible to ATP (Adenosine-Tri-Phosphate) which is more often known as the energy currency of the cell. When an inorganic phosphate is removed from ATP, energy is released which is then used for any function which required energy. Hence it is critically important for maintaining the energy levels in the cell.
ADP has only two high energy phosphate bonds. Another phosphate is added to the ADP by an energy consuming process to form ATP, which then acts as an energy store. Usually, the energy released during respiration is used for making of these ATP molecules from the ADP.