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
The latest changes to signatures involve pretty large changes to the ongoing size that the WSUS server will download during it's normal downloads of signatures from either an upstream WSUS server or from Microsoft Update. Prior to the package chaining implemention there were packages generated every signature release that contained a complete set (engine/current month signature base/signature delta from base to current time) for both x86 and x64 as well as packages that were similar but intended for clients that had not updated signatures in the past month. Those packages were about 55Mb for the complete and around 30Mb or so for the monthly package.
The short of the changes is that we will no longer have regeneration of these packages for each signature release but instead will use a feature of