True descriptors are Lace / Fingering / Sport / DK / Worsted / Bulky.
Within these weight classifications there is room for argument. For instance, some writers state that DK is the British yarnie's equivalent of the American's Sport. Others say that Aran is the British version of the American's Worsted.
Speaking of worsted, people often get confused with this term because it refers to a weight classification as well as a production style. In weight, it's the sweater weight and can be light worsted or just plain worsted, and it's usually a round fairly solid yarn. In production, it is a combing process which makes fibers lie parallel to each other before spinning, and the result is a stronger knitting yarn. People who prepare their own yarn from hand-carded wool, for instance, are not creating a worsted yarn, which is usually something done in a mill with larger machines. Of course, the hand-cranked carding machine will produce beautiful roving batt which is a true worsted product.
BUT ... back to actual classifications. You can find charts with classifications written out in the backs of many knitting magazines, or online. Some will give a suggested gauge, recommended needle size, and wraps per inch. I never trust those numbers, though, and always knit a swatch myself. That way I can choose what I want the fabric to look like, based on my own experiences. Here is a chart of generally accepted numbers for weight classifications. There is always room for personal interpretation, and some yarns will lend themselves to a weight because of how they bloom, yet not actually reach a weight classification. You'll notice that 'Sock Weight' does not appear, and that's as it should be!
- Lace Weight ......... 2300 yards per pound and higher
- Fingering Weight.. 1650 to 2300
- Sport .......... 1150 to 1600
- DK....... 1000 to 1200
- Worsted ........... 850 to 1100
- Bulky ... 500 to 900