Compound leaders are made up of segments or "compounded" of pieces of different diameters to achieve a particular casting result. A tapered leader is an example of a "compound" leader, compounded of segments that are decreasing in diameter. If you make your own leaders you can control the decrease or increase in size of and length of the segments. You can thereby create different rates of turn over and various loop sizes etc. Here is a place to start into the wide world of leaders:
http://www.flyfishusa.com/tackle-tip...t-leaders.html

Google the topic and spend as much time as you wish surfing leaders.