Fly-fishing lines are a special case because you cast the line, not the bait. Fly lines are made of a variety of materials and must be strong enough to hold the fish and heavy enough to cast. They must also be soft and flexible so they spool easily and can be handled without problems. Their surface must be slick so it goes through the guide on the rod easily.
Since leaders are used to separate the fly line from the fly, a fly line can be very thick and made to perform a variety of jobs. Some fly lines float to help keep the bait near the surface, while others sink to take the bait down. Some have lead cores to take a fly to the bottom fast. They often come in bright colors to make it easier for the angler to see them. Some fly lines can be used in both fresh and salt water but usually fly lines are made for very specific tasks.