Technically the ply of a yarn is about how its made, the number of plies (spun singles) that are plied (twisted or spun) together to make the yarn not how thick it is.
But here in Australia and in some other countries we are used to talking about ply as thickness. When I was a kid it wasn't problem as the available yarns were all made up of a certain number of similar strands and the 5 ply was made of 5 singles plied, the 8 ply was made of 8 plies and so on, and then thickness increased as the number of plies increased, making it easy for the two concepts to become one.
But in the US and more commonly now in Australia yarn thickness is specified by a category such as fingering or worsted and by a measure of thickness called WPI or Wraps Per Inch. You can buy little tools to measure the wraps per inch or you can just use a ruler or piece of dowel etc, (remembering that an inch is 2.5cm in metric), you can find more details on this page from About.com.
This can be quite confusing for people who are used to the 8ply 12ply etc system when they come across a 3 ply chunky weight like my favourite Malabrigo Chunky for example.
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Raverly has to translate yarn weights across all the different systems so they have put together a standard system as shown here.