An image backup is a complete copy of a hard disk or other media being backed up. The copy is complete in that it can be restored to a completely empty hard drive – as in a replacement hard drive after a failure – and the result is a hard drive that contains everything that the original did.
There are, of course, nuances to and even disagreement on the term’s meaning.
An image of a hard disk most commonly refers to a copy of all files, folders, and overhead information stored on the disk, including the information required to boot.
Another definition used less frequently is that an image is a copy of every sector on the disk, including those that are not used, as well as their physical layout. This is more commonly referred to as a “clone“.
An image by the first definition is all that is needed for backup purposes. When restored, the files are replaced on the hard disk, but not necessarily in the same physical locations as they were originally. (One positive side effect is that often a restored hard disk has no fragmentation.)
The second, when restored, places all files in exactly the same physical location as the original and restores all unused sectors as well, thus enabling deleted-file data recovery and potentially other forensic techniques.
Finally, the term image can be applied to either a partition, one portion of a hard disk, or to an entire hard disk. While making image backups of specific partitions can have value, only an image backup of an entire hard disk can be used to restore to a replacement hard disk.