A "raw" disk is a storage disk (if could be a physical disk, but usual is a LUN or a storage logical disk) that will be connect "directly" to the VM.
Usually VM disks are vmdk files that reside on a VMFS volumes on a storage disk.
In this case the raw disk is not formatted with VMFS but is formatted with guest OS filesystem.
With ESX there are different way to have a "raw" disk:
Raw Device Mapping (RDM): only for FC and iSCSI SAN
(RDM +) NPIV: only on FC SAN on ESX/ESXi 3.5 and 4.x (see also Configuring and Troubleshooting N-Port ID Virtualization)
VMDirectPath: only on supported controller and ESX/ESXi 4.0 (see also VMware VMDirectPath I/O)
Guest iSCSI disk: only on iSCSI SAN
Raw disk pro and cons?
Extend a RDM disk
How to check if VM use RDM?
A simple way is use RVTools.
But also Perl or PowerShell scripts can be used:
RDM on local disk
With the VIC or the vSphere client is possible create a RDM disk for SAN's LUN.
To use local storage with RDM (for example to connect and existing NTFS volume) the vmkfstools must be used from the CLI:
ESXi cannot use local disk RDM
Virtual disk performance
Performance Characterization of VMFS and RDM Using a SAN - http://www.vmware.com/files/pdf/performance_char_vmfs_rdm.pdf
Thin vs Thick - http://www.vmware.com/pdf/vsp_4_thinprov_perf.pdf