AssociatedItemCount versus ItemCount
Recently, one of my "Yes, but show me the TechNet article to prove it" co-workers asked me about the different between AssociatedItemCount and ItemCount properties in Exchange 2010 (and, of course 2007 and 2013) when looking at mailbox and public folder statistics. (We all have one of these co-workers, don't we?)
ItemCount reflects the total number of actual, viewable mail/calendar/contact/etc... items in the folder. Anything the user sees in the folder or mailbox.
AssociatedItemCount are the "hidden" objects associated with a folder or mailbox including rules, forms, form-type, Admin info, moderation information, views, auto-complete items, categories, etc...
Here are the relevant passages in the scripture.
MSDN Folder Associated Information
MSDN Content Tables
MSDN Folder-Associated Information Tables