Let’s first get the most common question out of the way.
The strings are the engine of your racket frame, where the “rubber hits the road”, and need to be changed regularly depending on how often you play. Strings are important as they can enhance your feel, comfort, and power. They will actually start to lose tension immediately once they come off the stringer. Certain types of string can lose up to 10% of their tension in the first night after restringing, and additional tension loss occurs each time you play. Some general “rules of thumb” for restringing frequency:
Next, the choice of strings and setup most suited for your play comes down to string material, string gauge, and string tension.
This is the gold standard for tennis strings, made using cow intestines. They offer the best playability, unsurpassed by any other synthetic strings. Top in power and comfort, with surprisingly good control for players with a fast swing. It is often used in a hybrid with poly to tone down the power when strung in modern frames, and to increase spin potential. This setup is used by many professional players- Roger Federer, Novak Djokovic, Serena Williams just to name a few.
A durable string that provides high durability, control and spin potential. Suitable for players with long and fast swings, and uses a lot of topspin. Not recommended for beginners, players with elbow discomfort or juniors.
This is the string that has the best playability apart from natural gut. It is very elastic and provides great comfort, feel and power. Durability is low, especially for players who use a lot of topspin. Flat, hard hitters will benefit from multifilament the most and also break it less often.
This is the most economical category of strings. It is a functional, all-round string great for recreational players looking for basic performance. It uses the same soft nylon material as Multifilament, without as many filaments, trading off power and comfort for more durability.
The most durable material for strings, Kevlar produces a very stiff and tight string bed. It is almost always used as a hybrid with a nylon string to reduce string bed stiffness. Good for control, but low in playability, power and comfort. Strongly discouraged for players with arm issues (eg. tennis elbow discomfort).
String tension recommendation varies depending on playing style and string type. Higher tension for more control, lower tension for more power and comfort. Most of the time we are stringing at 45-55 lbs (pounds) for the stiffer Poly strings and 50-60 lbs for softer Nylon/Multifilament. These are suitable tension ranges for Performance Graphite frames. For Midrange and Entry-level Composite or Aluminum frames, it should be 5-10 lbs lower. Players with arm issues (eg. tennis elbow discomfort) should also be using lower tensions for more comfort.
String gauge plays a part in playability vs durability. A thicker gauge gives better durability, at the expense of playability. Tennis strings have a gauge range of 15-19 (1.40mm-1.05mm) in general. It is best to remember by diameter, as different brands can define gauge slightly differently. For example, Babolat has 1.25mm as 17 gauge but Solinco has 1.25mm as 16L.
Finally, something to be said about hybrid string setups.
Strings hybrids are starting to gain popularity not only on the professional tour, but recreationally as well. The concept is simply the use of different string in the mains and the crosses to get the benefits from each. About 80% of the playability in your racket comes from the main strings, therefore the mains are usually the first to break. If you put a polyester string in the mains, that's the string that is doing most of the work and thanks to its durability, will increase the longevity of the string job as well as providing access to huge spin and control. Coupling the durable string with softer strings in the crosses will decrease the harshness that you'd get with a full polyester string bed. And at the same time, it will negate the poorer durability that a full bed of natural gut or multifilament gives.
By selecting different hybrid combinations of string, players can fine-tune the playability of their racket. Comfort, durability, liveliness and control can all be tweaked so you get a blend of the two and for a lot of players, that blend is preferable.