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, man-made version.
Professionals liken it to the difference between a Formula One racing car and a normal vehicle.
Top players like Andy Roddick will also have all their racquets restrung after every match or practice session.
Some will be strung very tight to give better control and spin, some much looser to give more power; so he can swap racquets depending on the way he is playing.
The rest of us settle for the tension that was pre-set in the factory, probably between 58 to 68 pounds.
One crude way to test if the strings on a new racquet are ok is to bang the strings on the butt of your hand to get a feel for their solidity.