ADP is a molecule which consists of the nitrogenous base adenine linked to the sugar ribose and which has a chain of two phosphate groups attached to the ribose in a linear fashion. ADP is used as an intermediate throughout glycolysis. ATP hydrolyzes to produce ADP and free energy.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.