The release of Oracle Database 12c is accompanied by extensive supporting collateral that details the new features and options of this major release. But with so much to read and investigate, where to start? If you don't have the time to pore through everything, then you may wish organize your reading in terms of the use case you're most interested in. If your interest is Database as a Service in private database clouds then may I suggest that you start here:
Accelerate the Journey to Enterprise Cloud with Oracle Database 12c
This paper describes the phases of the journey to enterprise cloud, and enumerates the new features and options in Oracle Database 12c that support each phase. Oracle Multitenant figures prominently, but it's not the only cloud-enabling topic: Oracle Database Quality of Service Management, Application Continuity, Automatic Data Optimization, Global Data Services and Active Data Guard Far Sync all deliver key benefits for delivering database as a service.
Further reading and research is suggested by the references included in the paper.
Happy clouding!