I've always wondered what 'worsted weigt' and 'fingering weight' yarn meant. As far as I know there is no similar system of knitting yarns in Europe (at least not in the Netherlands or in Hugary) Weird, ha?
In a pattern they just give one certain make of yarn (like 'Rowan Revive' or 'Katia Sirocco' ????) the needle size and the gauge. First of all, I don't know all the specific yarns so first I have to google them and even then I don;t have the slightest idea how thick they are. And then, somehow I tend to use smaller size needles than recommended so the needle size + an unknown make basically doesn't give me any information at all.
I think the specific names for certain yarn weights like below give already a hint of what kind of stitch size and structure to expect. (Of course, making a sample is always essential to determine the right amount of stitches to start with.)
Here is a link to an overview of knitting needle sizes (US, UK, metric and old UK-often used in vintage patterns)
Yarn weight overview
(source: Craft Yarn Council)