Higher Tension Considerations
In general, advanced tennis players -- those who hit with power, and have an aggressive style of play, and want more control over their shots -- tend to string their rackets with higher tensions. A tight string bed deflects less when contact is made with the ball, which results in better control of direction and ball speed. Choose a higher tension if you hit with a lot of spin because tighter strings "bite" into the ball better, providing greater spin potential.
Lower Tension Considerations
Choose a lower tension if you are a beginner; you don't hit with power; or you have a slow swing. A looser string bed deflects more at impact and returns more energy to the ball, providing more power with less effort. If you suffer from tennis elbow, choose a lower tension. The ball stays on the strings slightly longer with a softer string bed; the strings absorb some of the shock created at impact and less shock is transmitted to your arm. Some types of string, such as Kevlar hybrid, an extremely stiff and hard-on-your-arm string, should be strung at a lower tension for more comfort.