A codon is a sequence of three bases in DNA or RNA. A codon will code for one of the 20 commonally occuring amino acids, or for start or stop (TAA, TAG, TGA) codons which tell the transcription enzymes when to start and stop DNA transcription and translation. The start codon in humans is ATG, which codes for methionine, which is often removed after protein synthesis. The DNA and RNA codon system is almost completely universal, in that almost every organism uses the same codon to code for the same amino acid.

Here is a table of the 64 possible DNA codons, and the amino acids they code for.
First T      C     A     G   Last 
T     Phe   Ser   Tyr   Cys   T 
      Phe   Ser   Tyr   Cys   C 
      Leu   Ser   Stop  Stop  A 
      Leu   Ser   Stop  Trp   G 
C     Leu   Pro   His   Arg   T 
      Leu   Pro   His   Arg   C   
      Leu   Pro   Gln   Arg   A 
      Leu   Pro   Gln   Arg   G 
A     Ile   Thr   Asn   Ser   T 
      Ile   Thr   Asn   Ser   C 
      Ile   Thr   Lys   Arg   A 
      Met   Thr   Lys   Arg   G 
G     Val   Ala   Asp   Gly   T 
      Val   Ala   Asp   Gly   C 
      Val   Ala   Glu   Gly   A 
      Val   Ala   Glu   Gly   G

A number of exceptions to this do exist: several organisms, most notably archeobacteria, use more than 20 amino acids, (they have 2 extra), and several organisms use different start and stop codons.