Leaders and Tippet:
The purpose of the leader is to transition your terminal tackle from the bulky fly line to the fly that will be attached at the very end. Leader material is always a clear monofilament material or fluorocarbon and is virtually always tapered. The thicker end is attached to the butt section and the thinner end to either the fly or a piece of tippet material of the same diameter.
Types of leaders:
Tapered (knotless) leaders - by far these are the most common.and are made by melting down either nylon or fluorocarbon which is then extruded to produce a long strand of line with seamless transitions in diameter throughout the butt section, midsection, and tippet.
Hand-tied leaders (knotted leaders) - These are made by tying monofilament lines together according to a specific taper formula. Hand-tied leaders aren’t generally commercially available, anglers must tie their own. Here is one of the many leader formulas that is common.
Braided or furled leaders - are made of many small strands of line that are either braided or twisted together. Most braided or furled leaders are made of opaque or solid colored materials and only consist of a butt and midsection requiring standard monofilament tippet to be tied to the end.
The term butt section can refer to either the heavy end of the leader or a seperate 2 or 3 foot section of fairly heavy monofilament that is attached to the fly line with a nail knot or needle knot. The purpose of this short butt section is primarily to reduce the number of nail knots you need to tie when it is time to put on a new leader.
Tippet is used to add length onto the small end of the leader when, as the result of changing flies, etc. the small end of the leader gets shorter and shorter as knots are tied. As the leader gets shorter, the diameter of the leader increases so adding tippet assures that the fly is always attached to the diameter of line that is desired for that particular fly. Tippet is commonly available in sizes from 1X to 8X. The larger the number, the smaller the diameter.
Tippet size (diameter) is typically discussed in "X" size and relates to the diameter in thousands of an inch. There is actually a "rule" that will help in understanding how tippet sizing works..."the rule of 11's". The chart to the right will be helpful. So, it goes like this, if you take a given "X" size...say 5X and you subtract 5 from 11 you get 6...so the diameter of the tippet is .006 on an inch. Accordingly, 3X is .008" diameter, etc.
As the diameter gets smaller, it becomes less visible to the fish. It also has less strength so there is always a trade off...smaller diameter will theoretically hook more fish and it will also decrease your chances of landing the fish. Decisions...decisions...
Monofilament vs fluorocarbon - here is a good comparison of the two materials I found on theflyfishingbasics.com web site. "Monofilament has more stretch than fluorocarbon and it also floats on the water easier. Fluorocarbon has less stretch, resulting in more sensitivity and stronger hooksets. It also sinks faster in the water column and is more durable and abrasion resistant due to its hardness and the materials it is made of. Fluorocarbon is also near invisible to the fish. However, fluorocarbon is more susceptible to your knots breaking than monofilament and requires proper lubrication when you cinch your knots down. One other item is that monofilament is considerably less expensive that fluorocarbon. Each are adequate to be used when fly fishing though."