Peptide

Peptides are chains of amino acids. The boundary with proteins is a convention rather than a law of nature: chains up to roughly 50 amino acids are usually called peptides, and longer ones proteins. Insulin (51 amino acids) sits almost exactly on the line, which tells you how arbitrary the line is.

What makes peptides interesting pharmacologically is that the body already uses them as signalling molecules. Many peptide drugs are copies, fragments, or deliberately modified versions of a hormone the body makes on its own.