Somebody posted recently a query about yarn weights. Here is classification of
Elann company, displayed in thier homepage 
                      (http://www.cyberlink.bc.ca/~elann/elann.htm)
Hope, the fact I post it is not any violation of thier copyright.

Lace Weight Yarns     (Gauge = Finer than 32 stitches/4")
Baby Weight Yarns     (Gauge = 30-32 st/4")
Fingering Weight Yarns (Gauge = 27-29 st/4")
Sport Weight Yarns    (Gauge = 24-26 st/4")
DK (Double Knitting) Weight Yarns (Gauge = 21-23 st/4")
Worsted Weight Yarns   (Gauge = 19-20 st/4")
Aran Weight Yarns      (Gauge = 17-18 st/4")
Chunky Weight Yarns    (Gauge = 15-16 st/4")
Bulky Weight Yarns     (Gauge = 12-14 st/4")
Polar Weight Yarns     (Gauge = 10-11 st/4")

This table seems me also usefull - yardage of different yarns for different projects (from the same source)


  Gauge                         Chest       Chest        Chest      Chest     
 (St/4")                        46-48"      42-44"       38-40"      34-36"   

  30-32      (Baby Weight)       2800m       2600m       2400m       2200m    

  27-29        (Fingering        2600m       2400m       2200m       1800m    
                Weight)                                                       

  24-26      (Sport Weight)      2400m       2200m       1800m       1600m    

  21-23       (DK Weight)        2200m       1800m       1600m       1500m    

  19-20     (Worsted Weight)     1800m       1600m       1500m       1300m    

  17-18      (Aran Weight)       1600m       1500m       1300m       1200m    

  15-16     (Chunky Weight)      1500m       1300m       1200m       1100m    

  12-14      (Bulky Weight)      1300m       1200m       1100m       1000m    

  10-11      (Polar Weight)      1000m       900m        800m        700m     



Hope it helps.