A strand of DNA (deoxyribonucleic acid) has four components: Guanine or 'G', Cytosine or 'C', Adenine or 'A', and Thymine or 'T'. The strand of DNA will then match up with another strand, making it a double-helix (two spirals). The way they match up is the 'C's match up with the 'G', and the 'A's match up with the 'T's. This is hard to picture so here is a diagram.
(Above) This is actually just a table of how the parts of the DNA match up with the other parts. Below is a picture of what a double helix would look like.
As you see, all of the 'A's match up with all of the 'T's, and all of the'C's match up with all of the 'G's.
After a while this double helix will 'unzip' or split apart. The two halves become completely separate. Each half then links up with Messenger RNA (ribonucleic acid) or M-RNA, to form a new helix. The one thing that is different between two strands of DNA, and between one of DNA and one of M-RNA, is that with the M-RNA, all of the 'T's are replaced by Uracil or 'U's. Here is a table of what the lower strand of DNA from the top of the page would like, when it is paired with a strand of M-RNA.
A sequence of three pairs of chemicals is called a codon. This particular helix has three codons since it has nine pairs of chemicals. After the strand of DNA and the strand of M-RNA meet up, they go from the nucleus to the ribosomes. In the ribosomes the strands go through the process of translation, which is where the orders are given out by the newly-formed DNA.