A codon is a group of three nucleotide bases in either DNA or RNA that stands for a certain amino acid, which is the building block of proteins. The nucleotide bases are adenine (A), cytosine (C), guanine (G) and either thymine (T) or uracil (U), depending on the nucleic acid.
The genetic code, which is almost universal across species, is read three bases at a time. The code consists of 64 codons that make up the 20 amino acids making up proteins. Some amino acids are coded by several codons instead of one. In addition to the codons that represent amino acids, one codon, AUG, stands for the start signal, and the amino acid methionine and three codons, UAA, UGA and UAG, stand for the stop signal.