The energy is stored in the bonds that hold the ADP molecule together. The cell can only use ATP as a form of energy.
ADP is the molecule that will turn out in ATP, is the backbone of Adenosine triphosphate (ATP).
The ATP have 3 phosphates in the molecular structure, ADP have 2 phosphates, that phosphate means the realese of energy.
The ATP is the primary energy molecule of the cell, when it breaks in ADP the bond that link the phosphate 3 with the other 2 and 1 in the structure releases energy to cell's work.