VSS (Volume Shadow Copy Service) is a Microsoft Technology that allows taking manual or automatic backup copies or snapshots of computer files or volumes, even when they are in use. It is implemented as a Windows service called the Volume Shadow Copy service.
VSS operates at the block level of the file system.
Snapshots have two primary purposes: they allow the creation of consistent backups of a volume, ensuring that the contents cannot change while the backup is being made; and they avoid problems with file locking. By creating a read-only copy of the volume, backup programs are able to access every file without interfering with other programs writing to those same files.