Eddie:
Smcdermott is pretty well correct on all counts. There are basically 3 types of tippets that we all use, mono, fluro, and copolymer. The basic characteristics of each are as follows:
Nylon Mono: Pluses.....Very subtle, casts better, has a low specific gravity so it floats better with dry flys, has better knot abilities, less expensive, widely available, lower diameter has high breaking strength, has stretch so it absorbs shock better, long term shelf life .......Minuses, has a higher light refractive index (fish can see it), has a low specific gravity so it doesn't sink as fast with nymphs and streamers, is not as abrasive resistant.
Fluorocarbon: Pluses: high specific gravity, low refractive index, very abrasive resistant, very good with nymphs and streamers. .....Minuses: larger diameter, dont use it with drys, costs about 5 times as much, hard to tie a knot that wont slip, dosent stretch so be wary of hard takes, has some real quailty control issues about 5 years ago, rumors that it is affected by UV and has a shelf life.
Copolymer: Has a mono core and a flouro shell on the outside. They have attempted to marry the two substances and get the best of both worlds. The jury is still out on this one as far as I am concerned.
I have been through almost evert one of the flouros over the last 5 years. Thus far Rio gets 2 thumbs up on the copolymer and full flourocarbon as far as I am concerned. They have done a good job with their mixture. As a matter of fact the brown I have over here to the left was caught on Rio 6x flouro.
Segar must use a higher percentage of mixture, maybe even higher quality. But it is stiff so if you kink it, it's done. It has a tendancy to slip the knots. But at least it sinks like a rock and is virtually invisible. 1&7/8ths thumbs up
John