RDM (Raw Device Mapping) allows your VM to have direct access to a Disk / LUN - it acts as a special mapping file that acts as a VMFS volume tha proxies I/O between the VM and the disk.
When setting up an RDM with a VM there are two compatability modes - either Physical or Virtual.
Physical: Allows the guest operating system to access the storage device directly as if it were attached to a physical machine. This mode is useful when utilizing SAN-aware software on the VM - although this mode also has limitations - for example you are unable to take snapshots of the disk and you are unable to perform storage migration on the disk.
Virtual: Support full virtualization of the mapped device - meaning that features such as snapshotting, vMotion and so forth are fully supported. Although the real characteristics of the hard-drive are hidden and the guest operating system see's the disk how it would see a typical virutal disk file in a VMFS volume.
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