Adenosine triphosphate (ATP) is the main energy carrier molecule used living things. The difference between ATP and ADP, or adenosine diphosphate, is found in the number of phosphate groups, 3 for ATP and 2 for ADP.
Let's think of ADP and ATP as a battery - ATP represents the full battery, ADP represents a partially full battery.
ATP has a extra Phosphate group, which it gives off for energy where it is needed in the cell. Then it becomes ADP, ADP regains the phosphate through cellular respiration and becomes ATP again. The cycle repeats indefinitely.