Strings can make any racquet sing.
The right strings at the right tension are the most important aspect of any player's equipment.
There are many kinds of string for tennis, from natural gut (which is very expensive and very rare these days as it made from either cat or sheep intestines) to synthetic gut and nylon, there is something for everyone.
Most pros would use a natural gut or high quality synthetic gut string because of the incredible amount of "feel" those strings allow. Recreational players probably can't tell the difference between a $50 synthetic gut string and a $20 nylon string, so why pay more?
String is measured in gauge. 17 gauge is very thin, provides great feel and "bite" on the ball (allowing for more spin), but breaks easily, usually in less than two hours if you hit hard. 16 gauge is not as thin, but provides a good balance between feel, bite and durability while 15 gauge string is the thickest, most durable and best value for beginning to average players.
Most modern racquets have a little sticker on the inside of the throat detailing the ideal tension for that frame. String it loose if you want power; string it tight if you want control. A warning though: the tighter the string, the more stress you will put on your arm.