Yarn weights are: fingering, baby, sport, worsted, and bulky or chunky.
For best results, work an intricate stitch pattern with plain yarn and simple stitch patterns with novelty yarn. A novelty yarn can be such an attention-grabber, that an intricate stitch pattern often goes unnoticed when worked with it.
For a finished item that looks like the photograph, it is best to use the yarn recommended; if you are unable to find the yarn, or if you prefer to use another brand or yarn fiber, these simple guidelines will help you select a suitable yarn substitute.
The substitute yarn should work up to the same gauge stated in the pattern and should take approximately the same number of yards to make the item, but it may not weigh the same.