what is the difference between collagen and collagen peptides?

The fundamental difference comes down to size and structure.
Regular native collagen is a massive, tough, triple-helix molecule with a molecular weight of around 300,000 Daltons. Because it’s so huge, your body cannot absorb it effectively.
Collagen peptides (also known as hydrolyzed collagen) have been broken down through a process called enzymatic hydrolysis. This snips the massive triple-helix into tiny, short chains of amino acids weighing between 2,000 and 5,000 Daltons. Because of this extremely low molecular weight, peptides have near-perfect bioavailability and can easily enter the bloodstream.