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