ACGT stands for the four nucleic acid bases that make up DNA. The A stands for Adenine and pairs with the T, which stands for Thymine. The C stands for Cytosine and pairs with the G, Guanine. These four nucleic acids make up a creature's genetic code, or DNA.

ACGU stands for the four amino acids that make up RNA. RNA pairs up the same as DNA, except that Thymine is replaced with Uracil.