The specifications for tennis balls are defined by the International Tennis Federation (ITF). This means the ball must be a certain size and weight and made with certain materials. Tennis balls are made of 2-piece rubber cores that are cemented together. This is then covered with fabric, usually wool or synthetic felt. The seams between the two pieces of the core are exposed, but covered with elastic sealant.
The ITF defines the official diameter as "more than" 2 and 1/2 inches but "less than" 2 and 5/8 inches. The weight is "more than" 2 ounces but "less than" 2 and 1/16th ounces.
The rules also say that, when dropped onto a concrete base from 100 inches, the ball should bounce "more than" 53 inches but "less than" 58 inches. In order to achieve this, most balls are pressurized and filled with air.
Finally, the rules state that the "stiffness" of the balls, defined by how much they deform (curve in or out) at 18 lbs of pressure, must be "more than" .220 inches and "less than" .290 inches for forward deformation. For return deformation it must be "more than" .315 inches and "less than" .425 inches.
Because there is some "looseness" in the dimensions required for tennis balls, there is small, but noticeable differences between brands. Some balls are designed for hardcourt and grass, while others are designed for clay courts. The ball can vary by about 1/8 of an inch in size and 1/16th of an ounce in weight. Its rebound height can vary almost 10%. The stiffness, defined by the deformation under pressure, can vary over 5%. In addition, the rules do not say what material the cover should be, the thickness of the cover, or how tightly or loosely the fibers are woven.
An experienced player can instantly tell differences in balls, even though they all comply (follow) the legal specifications. Some balls are considered "lighter", "heavier", "harder", or "softer". Players find it takes more effort or less to generate the same ball speed. Some balls seem springier and bounce higher. The cover can seem coarser or thicker. With some balls, the cover fluffs up much sooner during match play, seeming to slow down in flight through the air.
Most of the major tournaments are played with different brands of balls, designed especially for the different court surfaces. One of the questions the team investigated at Key Biscayne was did any of the differences in balls, mentioned in the previous paragraph, really make a "difference" in professional tennis tournaments.