Cytosine (C) is a pyrimidine i.e., a nitrogenous base. A combination of a nitrogenous base (purine or pyrimidine) with a pentose sugar is known as nucleoside. Thus, the combination of cytosine with ribose sugar results in the formation of a nucleoside called as cytidine. Similarly, the combination of cytosine with deoxyribose sugar is called as deoxycytidine.