The genetic code consists of 61 three-base sequences, or "codons," in messenger RNA, each of which codes for one and only one of the 20 amino acids (There are also three codons that do not code for any amino acids. These codons are kind of like punctuation--the cell stops translating the messenger RNA when it reaches one of these codons, so they are callled "stop codons").
While there is more than one codon for most amino acids, each codon codes for just one type of amino acid. For example, the codons AUU, AUC, and AUA all code from the amino acid isoleucine, but the AUU codon never codes for a different amino acid than isoleucine.
The genetic code is nearly universal. While different species, and indeed different individuals within a species, have variation in the sequences of bases in their DNA (and thus in the messenger RNA that is made from their DNA), almost all of them use exactly the same code. For example, the codon AUU codes for isoleucine whether you find it in humans, daisies, or E. coli bacteria. This is what allows the human mRNA for insulin to be translated into human insulin in genetically modified bacteria.
There are a very few organisms that have a very few codons that code for a different amino acid than in most other organisms. Even in these cases, just one to three codons are different than usual and all the rest of them code for the same amino acids as in most organisms. One example is that in Paramecium species, the codons UAA and UAG code for the amino acid glutamine. In most organisms, UAA and UAG are stop codons. But all the rest of the codons in Paramecium code for the same amino acid as in other organisms.
