This is can be quite a complex thing in Australia where traditionally a ply was a specific measure. One ply was the finest and 10 ply was a heavy thick yarn and comprised of 10 strands. But, with imports of foreign yarns and new methods of production new ways needed to be used to express yarn weights. Now, and with many of the patterns being produced elsewhere it becomes necessary to find a way to easily determine how many metres of yarn you are getting and what would be it's uses.
It should be noted that there are some exceptions to the chart below - it's not a black and white fits all guide, but a general guide.
If you'd like a copy of the chart I've provided it as a pdf for you to download below.