The best players use strings made from animal gut, which is the lining of the intestine. This is very expensive and factory-made racquets are fitted with a cheaper, manmade version. Professionals liken it to the difference between a Formula 1 racing car and a normal vehicle. Some will be strung very tight to give better control and spin, some much looser to give more power; so he can swap rackets depending on the way he is playing.