Amino acids and the peptide bond
Amino acids are the units of the chain. In a peptide, the carboxyl group of one amino acid reacts with the amino group of the next. The link that remains is an amide bond, called a peptide bond in this context.
The sequence is written from the amino end, called the N-terminus, toward the carboxyl end, called the C-terminus. Changing the order of residues changes the molecule, even when the amino acids are the same.
Molecular structure
The backbone repeats the same pattern of nitrogen, carbon, and carbon. The side chains differ. Those side chains affect how the chain folds and how it interacts with water, other molecules, and a laboratory method.
A drawing of a chain is a model. It is not a measurement of a vial. Identity of a specific material requires analytical evidence for that batch, not a general diagram.
Peptides and larger proteins
Proteins are also amino-acid chains. The usual distinction is size. IUPAC-IUBMB nomenclature notes that authors often treat chains of fewer than about 50 residues as peptides and longer chains as proteins, and that the boundary is not fixed.
A short chain can still be biologically active in a published experiment. Length alone does not make a material a medicine, and it does not describe the purity or identity of a catalogue listing.