With 12c Oracle allows you to define container databases and pluggable databases. A pluggable database lives within / runs inside a container. You define Oracle memory allocations / sizes at the container level ( for shared pool / buffer cache etc ) and PDBs live with the definitions that are in effect for the container.
You are not currently allowed to carve out "dedicated" chunks of these critical oracle memory allocations for each PDB ( I can see that changing eventually perhaps ). So everyone is supposed to play nicely in their usage of these overall shared resources right?
Easy enough to demonstrate that one badly behaved PDB can grab way more than a fair share of resources. ( So download my complete Hotsos 2014 presentation maybe ? ).
Here is a quick peek at how it can look for the shared pool: