Alas, in the yarny world we may speak English, but when it comes to yarn weights (and crochet terminology) we are divided by a common language...
I say Aran, you say worsted, you say DK, I say 8 ply...you say single crochet, I say double crochet....
Here's a link to the US Crafts Council
Standards and Guidelines for Crochet and Knitting* document on yarn weights, terminology and more. A useful reference, recently updated to include jumbo weight yarn.
http://media.craftyarncouncil.com/files/CYC_YS_s_and_g_rev2015_6.pdf
Or if you just want yarn weights:
http://www.craftyarncouncil.com/weight.html
*crochet first! Woohoo!
Note: links are in US crochet terminology.