A disk image is a single file or storage device containing the complete contents and structure representing a data storage medium or device, such as a hard drive, tape drives, floppy disk, CD/DVD/BD and key drive, although an image of an optical disc may be referred to as an optical disc image. A disk image is usually created by creating a complete sector-by-sector copy of the source medium and thereby perfectly replicating the structure and contents of a storage device.
Some disk imaging utilities omit unused file space from source media, or compress the disk they represent to reduce storage requirements, though these are typically referred to as archive files, as they are not literally disk images.
Some backup programs only back up user files; boot information and files locked by the operating system, such as those in use at the time of the backup, may not be saved on some operating systems. A disk image contains all files, faithfully replicating all data. For this reason, it is also used for backing up CDs and DVDs.
Non-software type files can usually be backed up with file-based backup software, and this is preferred because file-based backup usually saves more time or space because they never copy unused space (as a bit-identical image does), they usually are capable of incremental backups, and generally have more flexibility. But for software files, file-based backup solutions may fail to reproduce all necessary characteristics, particularly with Windows systems. For example, in
Windows certain registry keys use short filenames, which are sometimes not reproduced by file-based backup, some commercial software uses copy protection that will cause problems if a file is moved to a different disk sector, and file-based backups do not always reproduce metadata such as security attributes. Creating a bit-identical disk image is one way to ensure the system backup will be exactly as the original. Bit-identical images can be made in Linux with dd, available on nearly all live CDs.
Disk imaging ensures hard drive recovery
Source: http://en.wikipedia.org/wiki/Disk_image
