The peptide bond is formed when a carboxylic acid group (COOH) of one amino acid and amine (N-H) group of another amino acid react together. It is simply an amide linkage (CO-NH)between two amino acids. Such large groups of peptide bonds between several amino acids gives rise to what we call a peptide chain. Many peptide chains combine together to form polypeptides and on further enlargement of the group of polypeptides, proteins are formed. Reference and further reading: Essentials of biochemistry by U. Sathyanarayana.