The principle called the Central dogma of molecular biology describes the process of translation of a gene to a protein. Basically specific sequences of DNA act as a template to synthesize mRNA in a process termed "transcription" in the nucleus. This mRNA is exported from the nucleus into the cytoplasm of the cell and acts as a template to synthesize protein in a process called "translation."
Three nucleotide bases specify one amino acid in the genetic code. Usually the first three bases of the coding sequence (CDS) of mRNA to be translated into protein are AUG (or ATG in DNA). AUG encodes methionine, and therefore the first amino acid of almost all proteins is initially methionine, although sometimes the methionine is removed after translation. The start codon is almost always preceded by an untranslated region 5' UTR.
Very rarely in higher organisms (eukaryotes) non AUG start codons are used.