Strings can be divided into to 4 categories - nylon, polyester, aramid and natural gut. Some strings can fall into subcategories of the mentioned categories but essentially they will be in 1 of the groups.

Nylon

Nylon is the most common material used to make racquet strings. This is because it is cheap to produce. In its simplest form a nylon based string will be a single strand, usual quite large (relatively speaking) and wrap in a number of smaller strands wound around it. An example of this would be Head Tournament 15L. This type of construction is used to create the most popular version of the string called synthetic gut. It is called synthetic gut not because it tries to play like natural gut (see below), but that it was the only alternative to natural gut for many years. The most popular and industry standard version is Prince synthetic gut.

There are many version of synthetic gut, some thicker for more durability, others textured for more spin/bite. However, there is a sub-category which has a different construction. These strings are called multi-filaments and are made made taking a large number, sometimes in the 1000's, of very fine strands of nylon. The aim of this type of string is to copy the properties of natural gut. With the amount of research that goes into producing this type of string, and the complexity of the construction, the cost is usually higher. An example of this string is Tecnifibre NRG2.

Polyester

Polyester strings are a fairly new type of string although their roots can be traced back over 30 years. The advance in string technology has brought this string to the fore-front of the market. Initially polyester string, or poly, was made with a single strand of the material and this is called a monofilament. Whilst this gave great durability and ideal for string breakers, it gave a harsh feel. Companies have introduced various materials into the string construction, and these strings are called co-poly. The additional materials usually help in giving a relatively softer feel, or increase the playability of these strings.

This type of string is now widely used by professional players as it allows the players to 'hit out' while retaining control. In essence the string reduces the power of the player. Use of poly strings have increased across all markets because they are very durable and can be low cost. However, the downsides of these strings are a harsher, stiffer feel compared to nylon based strings; and they don't retain tension as well meaning that their playability life can be short - upto about 10 hours before need to be replaced.

This string is often used in a hybrid with either a nylon or natural gut string. This helps offset the harshness of poly strings, and increase the power. It is advisable to avoid this type of string if you have arm, shoulder or wrist injuries as the harsh hit can produce vibration that are transmitted through the frame and into the players arm.

Aramid

Aramid strings are the correct name for string that commonly use kevlar as a base material. Kevlar strings are reknowned for being very durability. This was the go-to string for players who were frequent string breakers before the introduction of polyester strings. Kevlar should never be strung as a full bed as it is far to harsh. So, the normal use of it is in a hybrid with a nylon or natural gut cross string. This string is still used but only by those who want the ultimate durability.

Natural Gut

For many years racquets were only strung with natural gut. But with the introduction of nylon strings, the use of gut went down, and only used by professionals and those of discerning ability. However, natural gut strings offer probably the best attributes of any string. It gives the most power, the most control, the most comfort, and the biggest sweetspot. The downside of this string was durability, but companies have worked on coatings for the string that allow much better durability. Overall this string would be a great choice for most players. While more expensive, about 50% more that the top end nylon-based string, natural gut can offer an excellent all-round option. The actual life of the string, i.e. the length the string will perform at its optimum level, is much longer than any other string. Therefore, you may need 2 nylon restrings to 1 natural gut. 

Everyone should give natural gut a try.