A codon is a sequence of three nucleotides in DNA or RNA that either codes for a particular amino acid or tells the cellular machinery to start or stop using the code. A group of codons starts with the initiation codon. It then has codons in sequence that gives instructions on the amino acids to use to build a protein, and it then has a stop codon to signal when the protein assembly is complete. Normally, there is one initiation codon and three stop codons, and most amino acids are represented by more than one codon.
There are 64 possible codons, each composed of three of the four possible nucleotides, but only 20 amino acids are found in most biological systems. Accordingly, there is a great deal of redundancy, such as the amino acid valine with four codons. Only tryptophan and methionine have only one codon each. The codon for methionine is the same as the start codon, which is interpreted based on context.
The genetic language in codons is almost universal in life, but there are a few exceptions. One that is found in most forms of multicellular life is the mitochondria, the engergy production centers of eukaryotic cells, which have their own minimal genetic content and a slightly different genetic code.