An RDM is a LUN you present from your storage directly to your VM. => Deep Dive – Raw Device Mapping (RDM) | Virtualization Deep Dive
The filesystem will not be VMFS but the one used by the Guest OS.
It can be used for clustering solutions (Microsoft SQL Cluster, Oracle RAC, etc.. ) so two VMs can share the same LUN for high availibility.
About SDM, never heard about it. Is it VMware related?
RDM is a mapping file and is used to map a LUN directly to VM bypassing VMFS layer. This LUN can be formatted using any File System (NTFS or FAT32) without the need to format it using VMFS and placing VMDK file on top of it.
The mapping file is located with the virtual machine directory.
RDM has two compatibility modes:
VMware Knowledge Base Migrating virtual machines with Raw Device Mappings (RDMs)
There is no such thing as SDM in VMware.. and RDM is already explained by above comments