Collagen is a type of connective tissue, meaning it holds together or connects muscle tissue together. Initially very tough, collagen breaks down under heat, giving meat a tender, silky mouthfeel.
And finally we have elastin. This is another type of connective tissue and is primarily found in an animal's ligaments and surrounding muscle groups. It's stretchy and incredibly tough. Unlike collagen, elastin does not break down when the meat is cooked, and this is where we get gristle.
You get a lot of gristle in cuts of meat that come from the shoulders, legs, and haunches of an animal, like top round and chuck. It's easy to cut away because it's usually visible in clumps near where the muscle connected to a bone or as a silvery film across the surface of meat. If you don't manage to cut it away before cooking, it won't harm the flavor or texture of your cooked dish - it's just unpleasant for the person who gets that chewy mouthful!
Related: Why Tougher Meats Make Good Braises