The four bases in DNA are adenine, cytosine, guanine, and thymine.
These four are divided into two groups. The pyrimidines, the single-ring nitrogenous bases, is where thymine and cytosine is included. The purines, the double-ring nitrogenous bases, includes adenine and guanine.
Cytosine will only bond with guanine; they form three hydrogen bonds. Adenine will only bond with thymine forming two hydrogen bonds.