There are four nucleotides that make up DNA: Adenine, Thymine, Guanine and Cytosine. There are billions of them that make up one strand of DNA. Adenine always pairs with Thymine and Guanine always pairs with Cytosine.
Also, in the Rna during DNA transcription, Uracil replaces Thymine and pairs with Adenine instead.
Adenine (A) Thymine (T) Guanine (G) Cytosine (C)