What is it?
Offline access in Outlook Web App for Exchange 2013 lets users use Outlook Web App even when not connected to a network.
Offline access is newly available in Outlook Web App on the following web browsers.:
- Internet Explorer 10
- Safari 5 or greater (supported on Mac desktops only)
For more information about the offline user experience, see Using Outlook Web App offline.
--> Please see the rest on the blog : http://blogs.technet.com/b/exchange/archive/2012/11/06/offline-access-in-outlook-web-app-2013.aspx