The authoritative source for definition items = http://support.microsoft.com/kb/977939 

Some links to writeup these are getting a bit dated though as they do not detail Binary Delta Delta Changes or the new package chaining that occurs for serverside items:

http://blogs.technet.com/b/kfalde/archive/2009/02/09/understanding-fcs-definitions.aspx

http://blogs.technet.com/b/kfalde/archive/2009/03/05/wsus-fcs-definitions.aspx

Package Chaining changes to signature distribution

http://blogs.technet.com/b/clientsecurity/archive/2010/06/21/package-chaining-summary-from-kurt-falde.aspx