DNA is designed in such a way as to be intrinsically self-replicating. The rungs of double helix ladder are made of pairs of molecules called nucleotides. There are only four types of nucleotides and they are known by the letters A, T, G and C. There are only two possible ways one nucleotide can bond with another to make a rung. An A (adenine) can only fit with a T (thymine) and a G (guanine) can only fit with a C (cytosine). This is because of the shape of the molecules as depicted by the colored symbols on the right. The way these molecules bond is not by sharing valence electrons the way normal chemical bonding between atoms works. Rather, in organic chemistry (the chemistry of life), the shape of a molecule is the determining factor for whether bonding can occur or not. The entire pharmaceutical industry depends on this principle of organic chemistry where shape is most important.
The DNA is like a blueprint specifying how proteins are to be made. A protein is made of amino acids of which there are twenty occurring in living organisms. A protein is made by stringing together many hundreds of amino acids in what is called a polypeptide chain. The specific sequence of amino acids is what determines how the protein chain will fold up into a useful part for it destined purpose, like muscle or hair or bone. The sequence of amino acids is so important that any misplaced amino acid will render the protein ineffective. Since the DNA is the language that instructs the building of the protein, a mistake in the DNA is known as a mutation.
The language of the DNA is made up of words called codons which are a sequence of three nucleotides. This chart (click it for full size) shows how one or more codons translates to a specific amino acid. Since there are four possible nucleotides taken three at a time, that gives 3^4 or 64 possible combinations. Note that for reasons beyond the scope of this article, the U in the chart is equivalent to a T in the Nucleic alphabet. Note also that there are some codons which specify start and stop codes. Punctuation is part of the syntax of the DNA language! So a string of codons separated by start and stop codes represents a sentence in the language which specifies a single enzyme or protein. There is redundancy built into the language since in most cases more than one codon can specify a single amino acid. Perhaps this is because there is more going on here than what we understand. There is evidence that there may be multiple sets of information overlaid on top of the DNA than just protein building.