Yes, they confuse me too! Patterns that use a number (0,1,2,3,4,5,6,7) instead of telling you the yardage and grams of the yarn used or which weight of yarn to use such as lace, fingering, worsted, etc. When switching a yarn used in a pattern to something on hand, I like to compare the gauge, needles, yardage, and weight in grams so that the yarn chosen will match the pattern's yarn, because there can be quite a bit of variation between fingering weight yarns.
The chart below comes from the Craft Yarn Council and will hopefully help you demystify what yarn you need when the pattern calls for a number. We have also tried to help you out by including the number in our Drop Down YARN menu on the left side of the page.
Yarn Standards Chart from the Craft Yarn Council. Check out their website for more.