How do the nitrogen bases of RNA and DNA differ?

Three bases are identical in both DNA and RNA: adenine (A), cytosine (C), and guanine (G).

The fourth base in RNA is uracil (U); in DNA it is thymine (T). The difference between these two is small: U lacks a methyl group.

A and G are purines; C, T, and U are pyrimidines, which are smaller.