The term "disk-to-disk", or "D2D", generally refers to disk-to-disk backup. With D2D, a computer hard disk is backed up to another hard disk rather than to a tape or floppy. D2D is often confused with virtual tape, but differs in that it enables multiple backup and recovery operations to simultaneously access the disk directly by using a true file system.
Remote backup services are closely related to D2D backup as they are most often stored remotely on disk. The only major difference is that the data tends to be held at a remote location and these services are often provided by Managed backup providers.