Nylon

synthetic gut or nylon? Truth be told, synthetic gut is nylon. In fact, most of today’s “performance synthetics” are constructed of nylon, albeit a higher grade than basic nylon string. Today’s manufacturing processes produce nylon strings (or synthetic gut, if you insist) that provide a good combination of playability and durability. In the old days (wood racquet era), any self-respecting player used natural gut. Nylon was so bad that only beginners used the stuff. Today, 98% of non-professional players use nylon strings. It’s that much better. Other string materials include:

Natural Gut

the ultimate in playability and feel. Often overlooked due to it’s cost, natural gut is the best choice for players with arm problems or those who simply want the best. Formerly, the number one choice of ATP and WTA tour players. Now used more in hybrids, combining polyester mains with natural gut crosses. Natural gut gut offers maximum feel and control due to it’s low dynamic stiffness, which provides better ball “pocketing”, and a slight texture that provides more ball grab for enhanced spin.

Polyester

a very durable string designed for string breakers-not much power or feel. Polyester strings became very popular with ATP players, because it provides added durability, doesn't move and "deadens" the stringbed. While this isn't a desireable feature for most recreational players, it is for many of todays ATP and (some) WTA players. They're bigger, stronger, swing faster and use more powerful racquets than players from the past. Often used in hybrids, combining polyester mains with softer synthetic or natural gut mains. This offers the durability benefits of polyester, while reducing the stiff, dead feel. Also easier to string than 100% polyester. Not recommended for beginning players or players with arm injuries.

Kevlar

The most durable string available. Kevlar is very stiff and strings up very tight. Therefore, it is usually combined with nylon to reduce the string bed stiffness (Kevlar main strings, nylon cross strings). Still, Kevlar hybrids are the least powerful and least comfortable strings currently available. Players trying kevlar hybrids for the first time (from nylon strings) are recommended to reduce tension by 10% to compensate for the added stiffness. Not recommended for beginners or players with arm injuries.