There is only one file type that uses the .VMSD extension:
VMware Snapshot Metadata File
File created by VMware Workstation, an application used for virtualization; contains metadata used for storing information about a snapshot, which is a frozen saved state of a virtual machine at a point in time; used as part of a snapshot save and enables the snapshot to be loaded.
VMSD files are saved by VMware software behind the scenes and do not need to be opened manually. VM snapshots are saved and loaded from within the software user interface.
Updated: January 4, 2012