A SharePoint 2013 hybrid environment consists of an on-premises SharePoint 2013 farm and a tenancy in the new SharePoint Online. The integration of the two allows you to use certain resources from the two constituent environments to build unified solutions. These solutions then run seamlessly across the two.
A SharePoint 2013 hybrid environment supports these features:
As of the SharePoint Conference 2012, you can find information on SharePoint Hybrid Environments and links to downloadable white papers and poster here on TechNet.
Hybrid content for the SharePoint Conference 2012