Here, VSS = "Volume Shadow copy Service". Yes, Microsoft uses the acronym "VSS" for Volume Shadow Copy Service despite having an existing claim on that TLA via Visual Source Safe ie VSS.
You'd think they'd refer to it as VSCS, but no....
What I'm talking about is a facility in Microsoft servers (and client operating systems, but it's less important there) that's based on the Volume Shadow Copy Service where they can make automatic snapshots of their file systems on a schedule, and retain them until the total size of all snapshots reaches an admin-configured limit, at which point the oldest snapshot is dropped to make room.
The same underlying snapshot service is used to provide efficient image-based backup. In fact, on Win2k8 you can have the server maintain a bootable backup disk image of its self on a raw disk - I use an iSCSI volume on my Linux backup server. The server uses the volume shadow copy service to only update dirty parts of the image at each backup run. It's nice to have for a Windows-based server OS where unlike Linux/BSD you can't just rsync the whole file system contents to another box and expect to be able to boot it.
(For what it's worth, I use svn heavily, though am drifting git-wards now that I've started actually using it and discovered how seriously nice it is these days. You won't catch me near Visual Source Safe unless it's with an axe. In an amusing confluence of these two topics, I now maintain all my servers /etc in git and git-push them to the backup server every night, 'cos it's more convenient than Bacula when reverting changes).