The Craft Yarn Council (CYC) has created designations for yarn. While it’s not a perfect system, it’s quite good and many people rely on it and will say things like they’re “using a #4 yarn”. So, it’s good for you to be familiar with it.
We’ve put this on a separate page so you can easily print it for reference.
This is just a sampling of what’s in our Crochet 101 PDF!