The fly line is attached to the reel and is used to cast the fly. It is typically made of braided or coated materials and is much thicker and heavier than leader and tippet. The leader is a length of finer fishing line that is attached to the end of the fly line and to which the tippet and fly are attached. The tippet is the finest section of the leader that is used to help to present the fly in a natural manner.
The size of the fly line, leader, and tippet is typically chosen based on the size of the fly being used and the size of the fish being targeted. Smaller lines, leaders, and tippets are generally more delicate and can be more easily broken, but they also present the fly more naturally and are less likely to spook wary fish. Larger lines, leaders, and tippets are more durable but can also be more visible to the fish and may not allow for as delicate a presentation of the fly.