Original: The main ones are cytosine, guanine, adenine (DNA and RNA), thymine (DNA) and uracil (RNA), abbreviated as C, G, A, T, and U, respectively.
Simplified: They include cytosine, guanine, adenine, thymine in (DNA), uracil in (RNA) and xanthine and hypoxanthine (mutated forms of guanine and adenine). These are abbreviated as C, G, A, T, U, X and HX respectively. Because A, G, C and T appear in the DNA, these molecules are called DNA-bases; A, G, C and U are called RNA-bases.