Good question :D

A standard set of balls for the snooker table consists of 15 reds, 6 colours and 1 white / cue ball.The standard measurement of each ball is 2 1/16 inch in diameter. 

Historically, snooker balls were made exclusively from heavy ivory. Thankfully, these quickly became a thing of the past and were replaced before the Second World War by lighter Crystalate balls made from the crushed shin bones of cows.

Crystalate balls were subsequently replaced by Super Crystalate balls, made from entirely synthetic material. These were then replaced by the Aramith balls utilised in today's game.

Aramith balls are ideally suited to the demands of the modern game, as they are more reactive to spin and power shots, thereby presenting greater opportunities for enhanced cue ball control.

Taken from: 

http://www.fcsnooker.co.uk/beginners/the_balls/the_balls.htm

Traditionally, snooker balls were made of ivory. I understand that modern pool and snooker balls are made of a product called aramith. ;)

http://en.wikipedia.org/wiki/Billiard_ball

;)

In some cases they are made out of pixels, the size of which depending on the make and model of your monitor screen.