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From 2011.igem.org

Codon usage bias refers to the differences in the frequency of occurrence of synonymous codons in coding DNA. A codon is a
series of three nucleotides (triplets) that encode a specific amino acid residue in a polypeptide chain or the termination
of translation (stop codons).

There are 64 different codons (61 codons the amino acids and 3 codons express the stop signals). However, only 20 different
amino acids are transferred by these codons. Generally, every aminoacid have one codon for its expression.
On the other hand, an aminoacid can be encoded by more than one codon because of the overabundance in the number of codons.
The genetic code is considered as to be “degenerate” by the reason of such redundancy. Different organisms often
show particular preferences for one of the several codons that encode the same amino acid, hence a greater
frequency of one aminoacid will be found in the gene pool of the organism than expected by chance. The arisen of such
preferences is debated frequently area of molecular evolution.

Codon usage bias provides a balance between mutational biases and natural selection for translational optimization.
Optimal codons present the composition of the respective genomic tRNA pool in fast-growing microorganisms, such as
Escherichia coli or Saccharomyces cerevisiae (baker's yeast), optimal codons may help to achieve faster translation
rates and highly accurate and faster translation rates. As a result of these factors, translational selection is
expected to be stronger in highly expressed genes, as for the aforementioned organisms. The codon usage optimization is
normally absent for the other organisms that do not show high growing rates or have smaller genomes.,
Also, the characteristic mutational biases are determined for the codon preferences in particular genome of such organisms.
Homo sapiens (human) and Helicobacter pylori can be the examples of this case. . Some organisms show an intermediate level of codon
usage optimization for instance, Drosophila melanogaster (fruit fly), Caenorhabditis elegans (nematode worm)
or Arabidopsis thaliana (thale cress).

References
Ermolaeva MD (October 2001). "Synonymous codon usage in bacteria". Curr Issues Mol Biol 3 (4): 91–7.
Lynn DJ, Singer GA, Hickey DA (October 2002). "Synonymous codon usage is subject to selection in thermophilic
bacteria". Nucleic Acids Res. 30 (19): 4272–7