A Disk Image is a copy of an existing disk that encompasses everything from the files to the file system. Unlike simply copying all the files off a DVD or hard disk, a disk image is a perfect copy of the source.
Historically, disk image tools created a 1:1 copy of the disk (thus the disk image of a 10GB disk would be 10GB in size). Modern disk imaging tools generally use compression and the exclusion of non-written disk space to reduce the size of the final image (thus a 10GB disk might only have a 2.3 GB image).
Disk images are most useful for backups (you can create an image of an operating system hard disk in an optimum state, for example, to easily restore it in the future) and cloning (you can replicate a known good data or operating system disk multiple times for different machines).
For more information on disk cloning, check out the following How-To Geek articles:
- By Jason Fitzpatrick on 01/30/13