The description above is essentially on the money but I would add that tapered leaders which at first were all knoted sections of line was made of 3 sections; the butt the heavy line that transitioned the energy from the fly line to the leader, the taper a section of short pieces of mono that declined in diameter and finally the tippet. Most commercial fly leaders are tapered in the extrusion die and so may go from a 25lb butt section then tapered own to say 18 inches of 3lb tippet. Once that tippet collects a few wind knots you'll want to snip it off for your collection and add a couple feet of tippet using a blood or surgeons knot.Many people do still build their tapered leaders the old way uses several pieces of line and it offers a way to fine tune the leader to specific uses while store bought leaders are often sort of generalized for a variety of uses. If you are speyfishing with a dry or floated line you will want to use a tapered leader, or perhaps a floating poly leader. With my switch rods I build 14 foot leaders that start with 40lb maxima.