[stingraycharles]

There are many types of backups (e.g. replication, point-in-time replication, periodic incremental backups, periodic full backups) and not all of them project against the worst kind of issues, e.g. a software bug that accidentally deletes a whole bunch of data.

[fh973]

For Gmail, Google keeps backups of mail boxes and their transaction log. This way they can rewind the mailbox over a period of weeks.

It's basically a continuous backup.