A long chain of many amino acids is called a polypeptide. A polypeptide has hundreds or even thousands of amino acids linked together, while a peptide is a smaller chain of amino acids. As the name suggests, polypeptides are composed of multiple peptides.
A long chain of amino acids may or may not be considered a protein as well. In general, the basic chain of acids is considered a polypeptide until it takes its final three-dimensional form, which is functional. In addition, certain proteins are composed of both polypeptides and other molecular groups called cofactors or prosthetic groups. Proteins are responsible for an extensive array of biological tasks in most living organisms.