Classically, backups are taken then stored offline (e.g. on archive tape, on optical media, or on drives that are disconnected and put in storage between backups). Otherwise, if they are not 'cold' backups, they are still disconnected so if the main storage blows up, it won't impact the data in the backups.

Replication usually involves storage that is powered up and connected somehow to the same systems as the main storage; and any data that's corrupted on the main storage would propagate to the replicated storage.