Most players don't think about restringing until their string breaks, but in many circumstances, it's better to replace strings long before they hit their breaking point.
Every time a player hits the ball, his strings have to stretch and recoil. Over time, repeated stretching causes strings to lose their resiliency and playability suffers. Harder hitting increases the stress of each shot and wears strings more quickly.
The exact symptoms of dead strings are dependent on the player's racquet, technique, and choice of string. Most commonly, there will be a gradual but significant change in power from the stringbed, either increasing due to a drop in tension or decreasing due to a loss of string elasticity.
The ideal answer is to replace your strings when your racquet isn't playing as well as it did when the strings were new. A big hitter with soft co-polyester strings might notice the change in a matter of hours, where another player with a different style and setup might measure in months.
Synthetic gut is the most popular string among recreational players for its good all-around performance and exceptional value.
Synthetic gut strings are usually constructed from a nylon core wrapped in smaller nylon filaments. Various manufacturers have innovated on that basic recipe to produce a wide variety of performance characteristics.
These strings are pretty good at everything, but other types of strings will offer superior performance in specific areas.
Natural gut has been used for tennis for over a century and is still widely considered to be the best performing string available.
Natural gut is so named because it's manufactured from cows' large intestines (or less frequently, sheeps'). Its power, feel, and tension maintenance are yet unmatched by man-made materials.
Natural gut is the most expensive option for tennis strings, but because of its superior tension maintenance, it performs well right up until it breaks. It's possible that natural gut can outlast a number of sets of sythetic strings and provide competitive value.
Multifilament strings consist of hundreds or thousands of tiny fibers bundled together. Because of that, multis are usually softer and more elastic than synthetic gut. They offer lots of power and touch, but sacrifice durability to achieve it.
Polyester strings are popular among professionals for their low power and extreme spin potential. They allow players with long, fast strokes to hit precise and heavy balls.
Poly strings are known for durability, which is something of a double-edged sword. While they're very hard to break, they will "go dead" long before they reach a breaking point. When that happens, the stringbed becomes substantially stiffer and every performance characteristic drops off. Players who continue using the strings past that point will not only find their game suffering, but are at increased risk for wrist and elbow injuries.
Strings made from aramid fibers (Kevlar® is DuPont's trade name) are hands-down the most durable strings available. They stretch very little, so don't offer much in the way of power or feel. Most aramid users have moved to co-polyester strings for their more balanced performance.
As a general rule, a thinner gauge/diameter of string will provide more power, more spin, better feel, and less durability than its thicker counterpart.
Note: higher numbers are thinner when discussing string gauges.
Buying by the reel can usually save money, but there are a few questions to ask first:
Is a reel cheaper than buying individual sets?
Surprisingly, not all manufacturers discount string by the reel.
Will I love this string in a few months?
A reel is usually good for 16-18 sets of string. It takes most players a long time to go through that many stringjobs, so it's worth making sure that you're comfortable committing to the string.
If you go for the reel, just drop it off with your racquet when it's time to string. We'll cut off the minimum required length of string and do our best to maximize the number of stringings you can get from your investement.