Genetic code is the instructions in a gene that tell the cell how to make a specific protein. A, T, G, and C are the "letters" of the DNA code; they stand for the chemicals adenine, thymine, guanine, and cytosine, respectively, that make up the nucleotide bases of DNA. Each gene's code combines the four chemicals in various ways to spell out 3-letter "words" that specify which amino acid is needed at every step in making a protein. All living things use the same genetic code system, however, there exist some viruses that contain only RNA, which has the nucleotide base of uracil ("U") instead of thymine ("T").
The "word" combinations in more complex genetic strings if written out could literally fill a space as large as an encyclopedia.