The other thing that can confuse matters is the hook sizes. Yes, again they are different and US has letters and numbers. I like to use Metric (mm) because that's a standard measurement, so here is a chart to convert.
Most balls of wool you buy have the size crochet hook or knitting needle you need for that particular type printed on the label, but that may differ according to the pattern you are following and the tension you work at. It is a good idea to check you tension at the beginning of your project if size is important. It would be painful to reach the end of a large piece of work and find your one off, individually handcrafted, unique cardigan, is too small! Yarn also has the tension printed on the label in stitches/cm or your pattern usually state the tension for that particular project.
The other thing that varies is the name given to the weight and thickness of the yarn. Different information is provided on different websites and it gets a little confusing. One of the most popular weights in the UK is DOUBLE KNIT (DK) its equivalent in the US seems to be LIGHT WORSTED and in Australia its 8 PLY.
When Spinning WPI (wraps per inch) is used to measure the thickness more accurately so I have put another chart together as a rough guide, using WPI as the starting point.
Please feel free to download and print off the tables.