You can find the information in the 'glossary' of the UN Comtrade database. Please see below:

Commodity Classification: 

The complex nature of the basic customs and statistical needs makes it necessary to have a rather detailed commodity classification. The Harmonized Commodity Description and Coding System (Harmonized System, or HS), or extended versions based on HS, such as the Combined Nomenclature used by the countries that are members of the European Union provide such details. Classification using these nomenclatures is based on the nature of the commodity. However, for analytical purposes, such a division of products is not the most appropriate. Commodity categories more suitable for economic analysis are provided by the Standard International Trade Classification, Revision 3 (SITC, Rev.3), which classifies commodities according to their stage of production.

http://comtrade.un.org/db/mr/rfGlossaryList.aspx

HS2007 and SITC revision 4 are the most recent classifications.

http://unstats.un.org/unsd/trade/sitcrev4.htm