Item Category Group is one of the component which along 
with the Sales Doc Type, The Item Usage and Higher Level 
Item Category decide about the Item Category in a sales 
doc. 

for example we can ERLA as item cat group and carry out the 
header level pricing or we can LUMF and do the item level 
pricing in case of a Sales BOM. 

General item category group talks about further 
classification of a material. Like NLAG- non stock material 
can be used to identify bom header items, service items.

General Item category Group is basically determine business 
function of the material which is to be proceed in sales 
document e.g General item category Group can be Norm for 
services whichs indicate that this service is normal 
service for the organisation but Item Category Group must 
be LIES which is indicate the item is service with 
delivery. Item category Group also used to determine the 
Item category in sales document with the help of Item Usage 
and Higher Level item category.

Shambhu you are wrong.

item category:
it is an indicator that determines the characteristics of a 
document item

item category group:
it indicates how a material is processed in sales document 
and the sames is assigned  as a terms of determibation of 
account