Line & Leader

Line & Leader - Do you need a leader line? Do I need a leader with mono line? Should leader line be heavier than Main Line? How do you use a leader? Can you use fluorocarbon line as a leader? How do you tie a leader to the main line? How long should a leader be on braided line? What is the difference between leader line and fishing line? What is better fluorocarbon or monofilament?