From a Penn representative to the IDTA (Idaho Tennis Association). All you want to know about tennis ball construction. I'll bet you're just tingly, hey?...
In general, tennis balls are made up of a rubber core and a felt cover.
The core: Made from natural rubber which is a porous material, (this is why tennis balls go dead. The pressure within a ball leaks out over time through the porous rubber material. The time it takes depends on many different conditions, i.e. how hard a player hits, weather conditions, etc.) The cores themselves can be made to have different thickness' and formulas, thus creating a different feel of ball, (a very thick core ball will seem "heavy" or hard, while a thinner core will have a "lighter" or softer feel). Each tennis ball core is specifically made to produce a desired result.
The felt: Penn tennis balls are made with a cotton backing, and a combination of wool and nylon, (natural fibers are the best materials for durability and consistency). It's the proportions of the different materials and the way they are "weaved" together that makes one felt different from another. Varying the combinations will give a ball more, (or less), durability; more, (or less), "fuzz;" and more, (or less), playability. For example, the regular felt tennis ball is designed for clay, har-tru, carpet or grass courts, but is also used for indoor courts, (it cuts down on court "fuzz"). The women's Pro Tour uses this ball exclusively - no extra duty felt. The men switch off, depending on the type of court surface.
Production: The rubber core is pressurized and then the felt is glued to the ball, (by the way, the seam on the tennis ball is actually the glue that has come up through the two pieces of felt).
At Penn, we manufacture tennis ball products that are designed for specific individual needs: Premium balls for tournament & league use; Championship balls for all around use; and Mezzanine balls for recreational use.
Premium. Pro Penn & ATP Tour. Pro Penn is the exclusive ball for the USPTA and is available only through USPTA clubs and shops. It is made from interlocked wool fibers for longer wear and improved durability. ATP Tour is the official ball of the ATP Tour and is used for the ATP Tour tournaments. This ball also has a longer wearing felt in addition to a specially coated felt for moisture/stain resistance. Both of these products are high quality tennis balls with long lasting felt, pressure retention cores and superior playability characteristics for tournament and league players.
Championship. Penn Championship. The official ball of the USTA leagues and America's #1 Selling Ball. It is the standard by which all other tennis balls are measured. They can be purchased at any retail outlet and are designed to provide an exact mix of playability and durability. With a woven felt cover, this ball is designed with the frequent league and tournament players in mind - consistent performance day-in, day-out and available everywhere.
Mezzanine. Court One, Premier, Tribute. These balls use "non-woven" felt, (non-woven is a process that is less expensive than woven material and the end result is a somewhat less durable felt). They also have a rubber core better suited for the recreational player, (they may not be as demanding as league or tournament players). These balls can normally be purchased at large retail stores.
In summary, there's are ball for every player and type of court available. By and large, ball preference is a matter of personal preference determined by your style of play, the people you play with, your racquet, the strings you use, and/or the court surface you play on. To determine the difference in tennis balls, you have to play with different types of balls making sure that everything else is the same. For example, use the same racquet, court surface, string, but change from extra-duty to regular felt - then you will see the difference and determine what type of ball you prefer). After some experimentation, you'll be able to determine the best ball for your game.