DNA...

I know this post has come much later but I hope it helps anyways.
So what's DNA?
Two strands of linked nucleotides with one of the four bases adenine (A), thymine
(T), guanine (G) and cytosine (C).
Every cell in an organism has chromosomes in its nucleus which is made up of DNA strands coiled together with very high density. Think of this DNA as a string of letters from an alphabet. The english alphabet has 26 letters. Similarly, the DNA alphabet has 4 letters: A,C,T,G. Just like english has sentences made of words which are made of the 26 letters, exactly the same way, DNA strands have strings made of "codons"(explained in a moment) which are made of the 4 letters.

What are these codons?
Codons are equivalent to the words in English. But English has words of different lengths . Heres where codons differ. They ONLY consist of 3 nucleotides(A,C,T,G). Each codon then after transcription (conversion of DNA to RNA) gets translated by a complex mechanism to amino acids. Again using analogies from English language, like every word has a specific meaning (stupid eg. you cant use "man" when you mean "car"), every codon can be translated only to a specific amino acid. Strings of amino acids form proteins. Again, since there are 4 nucleotides and 3 letter long codons, there are 4^3 = 64 possible codons. But there are only 20 amino acids(more about them later) thus there is some ambiguity. Like man, male, fellow refer to the same thing, many codons code for same amino acid. Apart from the 20 amino acids, there is one START codon and 3 STOP codons. The codon table is given below (taken from [1])



What is transcription?
Transcription is the process of converting DNA to RNA. RNA has the 4 letters A,C,G,U in its alphabet. More about transcription later.

Fig: from Bioinformatics: from data to biological knowledge by Dena Leshkowitz

References:
1. http://www.cs.cmu.edu/~blmt/Seminar/SeminarMaterials/?N=A

Further Reading:
1. Molecular Biology by Robert Weaver for details on transcription and translation.