There are two categories of backup: system backup and file backup. System backup copies everything verbatim on the hard drive onto an external storage medium, including the operating system, applications and data. If the drive fails, the computer can be restored to its former state. File backup copies only data files onto an external storage medium. If the drive fails, the OS and applications have to be re-installed. Following are the various types. Full and Differential Backups A full backup copies all selected files. A differential backup copies only files that have changed since the last backup. Incremental and Delta Backups When each revision of a file must be maintained, an incremental backup appends selected files to the backup medium and does not replace existing backup files. A delta backup is similar to an incremental backup but copies only the changed data within the selected file, not the entire file (it backs up the differences). See deduplication, backup software and cloning software.