Peptides are chains of amino acids joined by what chemists call amide bonds, although biologists typically call them peptide bonds instead. Large peptides are called proteins and are ubiquitous — all forms of life on Earth have proteins. The term peptide is usually though not invariably reserved for shorter chains of amino acids such as the hormone glucagon or the small tripeptide glutathione, both found in the human body.
The amide bond in a peptide features a carbon atom with a nitrogen atom and an oxygen atom attached to it. You can find a drawing of the structure of a typical peptide bond in the Resources link below. You can draw the peptide bond in two possible ways; you can draw a double bond between the nitrogen and carbon atoms with a negative charge on the oxygen atom, or you can draw a double bond between the carbon and the oxygen with a single bond between the carbon and the nitrogen atoms. These two alternate structures are called resonance structures.
The true structure of the peptide bond is actually a hybrid of the two resonance structures. This hybrid is closer to the resonance structure with a double bond between the carbon and the oxygen atoms — in other words, this structure is the more significant contributor — but the other resonance structure plays a significant role also. In particular, the bond between the nitrogen and the carbon behaves like a double bond in the sense that neither atom can rotate with respect to the other.
The constraints set by the nature of the peptide bond help to dictate the 3-D shape the peptide adopts. Interactions between chemical groups on the amino acids in the chain also help to determine which parts of the peptide face outward into the water and which face inward, toward the center of the peptide. The shape of a protein or peptide is a critical factor in determining its function, so a great deal of ongoing research is dedicated to understanding how peptides and proteins fold up once they’ve been synthesized by the cell.
Both polypeptides and peptides are chains of amino acids; the only difference is in the size. There is no hard-and-fast dividing line between a polypeptide and a peptide, although biologists usually reserve the term peptide for smaller chains with a few dozen amino acids or less. You can find both peptides and polypeptides anywhere you find life on Earth. All organisms from trees to bacteria synthesize and make use of peptides and polypeptides alike.