Natural Gut is the oldest type of tennis string and often the choice of professionals and advanced players. it offers an unrivalled combination of a lively feel and amazing power while holding tension perfectly over time. Natural gut is a completely natural product (cows intestines) which results in a higher cost. These strings can be less durable than other synthetics so if you are likely to break strings, alternatives should be advised.
Natural Gut pros:
Holds tension perfectly over time
Remains soft at high tensions
Natural Gut cons:
Less durable than other types of string
Doesn't perform as well in wet or humid weather