DBRM is available on the Database layer on any platform to prioritize resources like CPU and parallel degree, but on Exadata only it can be used to prioritize I/O.
IORM works on Exadata Storage Servers only.
DBRM is used for Intra-Database plans as well as for Category plans that span multiple Databases (together with IORM).
IORM is used for Inter-Database plans and for Category plans (together with DBRM).
Adding One more additional Point here
Database Resource Manager (DBRM) is tightly integrated with Oracle Exadata Storage Server I/O Resource Manager (IORM).
In Oracle Exadata, DBRM and IORM work together to ensure that I/O resources allocated depends on Administrator defined priorities in the system
IORM feature is available only on Oracle Exadata Database Machine.