Not all strings are created equal
Tension
Within the recommended tension range, lower tensions offer significantly less stress on the arm and slightly more power, and higher tensions offer significantly more control and slightly better spin.
Remember that you don't hit the balls with your precious racquet but with your strings. So, selecting the right one will affect how comfortable you are with your shots. Unfortunately, there's not a perfect string that fits every player and in general, there's a trade-off between power and control when it comes to selecting a string and it's tension.
Gauge
The higher the gauge number, the thinner the string and vise versa. Half-gauges are represented by the letter L. Thicker strings (15, 15L, 16 gauge) provide less spin, elasticity, comfort and feel but are more durable; thinner strings (16L, 17, 17L gauge) are less durable but provide better bite, elasticity, comfort and feel.
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String Pattern
With a dense pattern in your racquet, strings last longer. A wide pattern is great for top spin but there is more movement between the main and cross strings. This causes the strings to chew each other and break. In this case, restringing costs can lower down by choosing a string with thicker gauge. Polyester is a good choice too.
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Material & Composition

Gut - This are made from cows gut in a complicated process, so this string is super expensive but has great elasticity, tension stability and liveliness. Because of the high price, gut is not recommended for the average recreational player. Not to mention it is not very durable. Gut is also very sensitive to moisture.
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Synthetic Gut - They give the user different charateristics such as durabiliy, spin, feel, power etc.. Synthetic gut strings can be classified in:
Nylon - A good all-around string category. This is the basic, and most popular string choice in tennis. It also happens to be one of the cheapest. It has a crisper feel compared with Multifilaments, good, but not as gentle on the arm as Multi or Gut. It's reasonably durable and holds tension well. A good category of string when you're looking to save money.
Polyester & Kevlar - This is the durability category; the choice for hard hitters, string breakers, and people without arm problems. Expect harsher hits (very harsh with Kevlar) with above average control. Kevlar (aramid fiber) is extremely durable and holds tension very good, but not recommended as the only string in your racquet - hybrid use only. Poly has much more playability, it's use is not limited to hybrid applications like Kevlar, and Poly holds tension fair. A good category of string when you're looking for maximum durability and control.
Multifilament - The top category after natural gut. Best overall playability, gentle on the arm, holds tension fair. Second most expensive string after gut. A good category of string when you're looking for arm friendly, power and control.
Textured Strings - These are the strings that have an added raised band to give the string texture. The idea is to produce more spin.
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Tension Control Power Comfort Feel Durability
Lower Less More More More More
Higher More Less Less Less Less
Gauge Bite Elasticity Comfort Feel Durability
Thinner More More More More Less
Thicker Less Less Less Less More
