Yarn Weight Guidelines
Since the names given to different weights of yarn can vary widely depending on the manufacturer and/or the country of origin, the Craft Yarn Council of America has put together a standard yarn weight system as a general guideline to follow. Look for a picture of a skein of yarn with a number 1-6 to figure out its weight. You will see the gauge printed as well - gauge is usually given over Stockinette stitch. The information in the chart below is taken from www.yarnstandards.com.
If you substitute yarn, be sure to select a yarn of the same weight as the yarn recommended for your project. Even after checking the recommended gauge on the yarn label you plan to substitute is the same as for the yarn listed in the pattern, make sure to make a gauge swatch to see. Use the yarn weight chart above as a general guideline for selecting a yarn to substitute.
A note about the fiber content: it is also a good idea to substitute a similar fiber to make sure the shape of the project comes out correctly. For example, silk drapes differently than cotton or wool, and vice-versa.