a codon is the 3 nucleotide sequence which codes for an amino acid, stop or start signal in translating a protein.
example:
common mRNA start codon is AUG
(Adenine, Uracil, Guanine)
one of the codons for the amino acid Alanine is : GCU
from general genetic code table for RNA.
codons will be abreviated by 3 nucleotide letters A, G, U, T, C
Uracil is only in RNA codons
there are also DNA codon tables
these tables are a general guide as different organisms may have special codons they utilize.
below is a table of the twenty most common amino acids that make up proteins, below each are the codons generally found to code fore them.