In the Universal Yarn Finder I have made an arbitrary division of yarn into five classes. The classes are defined according to weight and knitting stitch gauge. They are:
Class A light weight fingering -- 7 or more stitches per inch
Class B medium weight sport -- approximately 6 stitches per inch
Class C heavy weight worsted -- approximately 4-1/2 to 5 stitches per inch
Class D bulky weight -- less than 4 stitches per inch
Class E super-bulky weight -- approximately 2-1/2 stitches per inch
The actual size photo gives you a visual reference to what kinds of yarns fit into which Classes.