Cytidine triphosphate is a molecule which consists of the nitrogenous base cytosine linked to the sugar ribose and which has a chain of three phosphate groups attached to the ribose in a linear fashion. CTP is required for the synthesis of DNA since it is a direct precursor molecule.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.