Cue sport involves balls of different sizes, colors, coatings and patterns etc that determine friction coefficient, resilience and hardness. Earliest pool balls were made of clay, wood and later ivory but contemporary balls are manufactured by plastic materials that are not prone to chipping or cracking. 
In carom billiards, balls are not numbered and 61.5 mm in diameter. Here four/three balls are played with two red object balls and one white/yellow cue ball for each player. American style pool balls are larger than British pool balls and are widely used worldwide. These are 2.25cm in diameter with 0.005cm tolerance, colored and numbered. Typically there are nine-ball or eight-ball games played by fifteen balls in pocket billiards. In a British style blackball game 5.08 cm object balls and 4.76 cm cue balls are used but without any numbering. Twenty two colored balls of same weight and of 52.5 mm diameter (tolerance 0.05mm) are played in snooker.
In Russian Pyramid and Finnish Kaisa games, all-white numbered object balls are played by yellow/red cue balls. These balls are 68 mm or 72 mm in diameter. Bumper pool requires eight object balls and two special balls of approximately 52.5mm diameter. Also there are some special training balls for beginners and novelty balls like Blacklight, Boble ball, Tag ball etc.