Although there are various brands to choose from, there are basically two types of balls you can buy-pressurized and pressureless.
By John Levey / Copyright: TENNIS July 2006

If you are playing a match, the lively bounce and greater playability of pressurized balls make them the only way to go. But pressurized balls won?t survive longer than a few sets. Players who collect balls for practice or for use in ball machines would be wise to go pressureless.

Since the pressure inside the ball is equal to the air surrounding it, there is never any change in playing characteristics due to the diffusion of gas over time. The thicker rubber core of a pressureless ball provides all the ball?s hardness and resiliency, and you will wear out the felt before you deaden the ball. Pressureless balls have a lower bounce, heavier feel, and are tougher on the arm, but their durability is far superior.