Nucleotides can be divided into two catergories, purines and pyrimidines. Pyrimidines are named so as they are derived from the six membered ring pyrimidine . Examples of pyrimidines are cytosine, thymine and uracil (which is only present in RNA). These bond via hydrogen bonds with their complementary purine bases when in DNA's double helix form. They also exist in RNA, where thymine is replaced by uracil.
Pyrimidine refers to the structure of the molecule whereby it is a 'six-membered pyrimidine ring'. Examples of molecules which possess this structure are the nitrogenous bases Thymine, Cytosine and Uracil .