Introduction SharePoint 2013 (and previous versions) uses a client side “token” to validate posts back to SharePoint to prevent attacks where the user might be tricked into posting data back to the server. This token is known by many names; form digest or message digest or request digest. The token is unique to a user and a site and is only valid for a (configurable) limited time. When building Apps or customizations on top of SharePoint, especially using patterns such as Single Page Applications (SPA) or using frameworks such as knockout.
Introduction SharePoint 2013 introduces a new model on how Site Collections are upgraded using the new Deferred Site Collection Upgrade. To keep it simple it means that SharePoint 2013 can run a Site Collection in either 2010 mode or 2013 mode, and SharePoint 2013 contains a lot of the SharePoint 2010 artifacts (JS files, Features, Site Definitions) to handle this. When you’re doing a content database attach to upgrade from 2010 to 2013, only the database schema is upgraded and not the actual sites (by default).