DATP (Adenosine TRIphosphate) has one more phosphate group and is higher in energy than ADP (Adenosine DIphosphate). TRI means three, DI means two, so the names of the molecules tell you there's one more phosphate in ATP than in ADP.
That extra phosphate makes ATP higher in energy (when the cell uses that energy to do something - contract a muscle fiber, for instance - the ATP's extra energy is used and it gets converted to ADP).
