Developer training | apps for Office and SharePoint
http://msdn.microsoft.com/en-us/office/apps/fp123626.aspx
Apps for SharePoint overview
http://msdn.microsoft.com/en-us/library/fp179930.aspx
How to: Create a basic SharePoint-hosted app
http://msdn.microsoft.com/en-us/library/fp142379.aspx
Get started developing apps for SharePoint
http://msdn.microsoft.com/en-us/library/jj163980.aspx
SharePoint 2013 Apps, by Salman’s Blog
http://blogs.salmanghani.info/sharepoint-2013-apps-2/
Introducing Apps for Office 2013 and SharePoint 2013
http://blogs.msdn.com/b/chaks/archive/2012/08/16/introducing-apps-for-office-2013-and-sharepoint-2013.aspx
SharePoint-hosted app model for SharePoint 2013
http://www.bing.com/videos/browse?mkt=en-us&vid=cdeb6d62-bb64-42df-8fbd-c0420957531e&from=sharepermalink&src=v5:share:sharepermalink:&from=dest_en-us
An Overview of SharePoint 2013 App Development Model
http://www.simple-talk.com/dotnet/.net-tools/an-overview-of-sharepoint-2013-app-development-model/