Some ping pong balls are made of plastic polysterene or poly and there are those made of celluloid. It is recommended to go for poly balls because the ITTF or International Table Tennis Federation as the standard option for official events considers these. While celluloid and poly balls play the same during a game, poly ping pong balls are easier to transport safely and environmentally friendly at the same time. The celluloid ping pong balls come with a cheaper price tag and are slightly more durable and it is best to stick with the standard plastic poly option as suggested by the ITTF.
One more thing to remember is that celluloid balls measure 40mm while plastic poly balls are a bit bigger that you can sometimes see them listed with a size of 40+. If you see a ping pong ball and you are not sure what material it is made of, you can always check its inscription. Poly balls are printed with 40+ while celluloid ones have 40 or 40mm printed on them.
When comparing ping pong balls, you will notice that all of them have a star rating ranging from 1 to 3 stars. With this rating system, 3 stars mean the highest quality while 1 star is of lowest quality. It means you have to look for ping pong balls with 3 stars. Training balls as well as 1-star and 2-star balls are much less durable and could break faster compared to the 3-star ones.
1-star balls are soft and perfect for youth play. The 2-star ones are of medium quality and slightly firmer than 1-star balls. These are perfect for multi-ball practice, for game improvement, or for use in training robot. Finally, 3-star balls are of the finest quality and firmness. These balls are what you have to use for game play, with the rating level being the sole one approved and recommended for tournament play. In case you are only practicing several multiple-ball drills such as serves and smashes, you can opt for good training balls or the bulk 3-star pack solely for the purpose.