What is VSS and what is it used for?
Solution 1:
Volume Shadow Copy Service.
It takes a snapshot of files so they can be backed up even while they're in use. It ensures that the file is not modified between the start and end of the backup, but can't ensure that the file is in an actually usable state should it need to be restored (for instance, a database file may be in mid-row modification when the snapshot is taken so the file itself is in a consistent state, but the application using it chokes on the half-finished edit).
Solution 2:
It's the Volume Shadow Copy Service.