There are 4 main types of strings which are used by 99% of the tennis community, they are –
Polyester strings , Synthetic gut, Nylon strings , Multi-filament strings and Natural Gut.
Nylon strings are basically a mono-filament string made out of nylon, they are the cheapest strings in the market.
Synthetic gut strings are an upgrade from nylon strings, and is still made of nylon but its structure is different.
It consist of a mono-filament core but with additional wrappings(1 or 2 layers) of multi-filaments over the core. This enhances the feel and qualities of the string.
Multi-filament strings are the next upgrade. Still made out of nylon but now there is no mono-core, the whole string is made out of thousands of thin fibers of nylon. It is also usually coated with special chemicals to make it more resilient. These enhances the feel and qualities of the string much more. These string are pretty expansive as the manufacturing process is not easy, not many string companies can produce such strings. Multi-filament strings are the closest strings to natural gut.
Natural gut strings are made from cow’s intestines. These are the most expansive strings in the market ( costs about 3-4 times more then a Multi-filament ). This is the string that offers feel and qualities which all the above mentioned strings strive to emulate.
Natural Guts > Multi-filaments > Synthetic Guts > Nylon strings.
It takes roughly 3 cows to make one one natural gut string. A small part of the cow’s intestine called the serosa is used.
And lastly – Polyester strings.
These are stiffer strings that has superior durability, control and spin. More than 90% of the pros have this string in their racquets as hybrid or full. These are the strings that has opened up a whole new dimension for the modern tennis game and is one of the main reason why the pros can hit the balls the way they do today.
These are mono-filament strings but comes in a huge array of shapes and gauges.
Superior 3rd generation polyesters has replaced the older polyesters in today’s industry. And there is a huge amount of 3rd generation polyester strings with different feel.
Regarding this string, its a huge topic. Click ‘here‘ for more info.