Bowling balls are made out of polyurethane, plastic, resin and a thick outer coating that allows the balls to roll smoothly. These materials are combined and are used for different reasons depending on the bowling ball.
In the past, bowling balls were made exclusively of heavyweight rubber. These balls were extremely bouncy and did not have much accuracy when rolling down a bowling lane. They were also not heavy enough to effectively knock down all of the pins in a game of bowling. They were then covered in a resin material that would allow them to roll smoothly and would reduce the amount of bounce after they were rolled.
Over the decades, especially in the middle of the 20th century, bowling balls began to become greatly improved. Different materials and methods were tested and used. Bowling balls often got resin coverings and quickly became made out of polyurethane that is more conducive to rolling down a lane than rubber is. They continue to improve and there are many methods for creating effective bowling balls. The inside of the bowling ball is often made out of the polyurethane material while the outsides of the bowling balls are often covered in a slick plastic or hardened resin material.