A tennis racquet is made up of essentially 2 important components the frame and the strings. As such, each component plays a very important role in how a tennis racquet plays. The frame usually provides the core characteristics of the racquet with the strings providing the fine-tuning or subtler characteristics. Eg. A stiff powerful frame such as the Babolat Pure Drive can be fine tuned to have a bit more feel and comfort with natural gut strings. Given that there are hundreds of different strings available to today's tennis players which allows many choices in string setups for fine tuning one's racquet performance.
This is an economical choice in synthetic strings which provides good playability with reasonable durability. It is usually constructed with nylon core in the middle and with nylon wraps on the outside for ball bite and durablility. These strings do move quite a bit especially as they loosen up over time.
Also constructed with nylon but with many small individual fibers twisted in various configurations. Multifilaments are meant to replicate the feel and playability of natural gut using synthetic materials. Multifilaments do play softer with more feel than synthetic guts/nylons but not quite as good as natural gut. They cost less than natural gut but more than nylon strings due to their complex manufacturing and construction. Due to the use of fine filaments, these strings tend to be more fragile and less durable.
These strings are relatively new and for many pros is an essential part of their equipment. The reason for this is that polys' inherent stiffness and limited elasticity provides more control with more spin potential. This inherent stiffness also ensures limited string movement as well. Many top pros use polys in hybrid setups to take advantage of its control and spin qualities.
Unfortunately this inherent stiffness and low power also translates to greater harshness leading to arm pain and even tennis elbow. Polys are also very low powered and lose their playability relatively quickly and thus needs to be restrung more often than other string types.
Considered to be the ultimate tennis string, natural gut has excellent playability, good power and feel without too many tradeoffs. It is also the most arm-friendly string giving excellent comfort and feel. The only disadvantages with natural gut is its vulnerability to moisture which can limit its durability and its high cost. Otherwise, it is provides the most positive attributes of all the string types but it is also the most expensive.
Recently many pros have begun to use hybrid stringing to get the ideal characteristics from their string beds. A hybrid string job simply means a string job consisting of 2 different strings. You can a hybrid of poly/poly, poly/nylon, poly/multifilament, gut/poly etc. The combinations are almost endless! A hybrid string job is a part of the "fine tuning" of a string job. An example where you might consider hybriding would be if you enjoy the spin and control of a specific poly string but it feels a bit too harsh on your arm. To retain the best qualities of this poly while making it more comfortable one might put in a softer nylon/multifilament/natural gut. Generally, 75% to 85% of a stringbed's main characteristics such as playability and spin come from the main strings. The softer crosses will help with the comfort of the stringbed.
Finding the right tension for your string job is almost as important as in the string selection. The reason for this is that all string types have differing elasticities and tension holding abilities. Natural gut for instance has the greatest amount of elasticity and retains it very well and over a long period of time. Contrast this to polys which inherently have very limited elasticity and loses it quite quickly. All strings have an upper limit of tension at which point the elasticity will be reduced permanently.
Finding the proper tension for the strings which you have selected takes some time and patience. It will all come down to play testing and tweaking the string tensions each time you restring.
In general, the lower the tension the better the power and feel. The higher the tension the more control you will obtain.