Disk image files contain an exact copy of the data from a computer hard-disk, CD or DVD. These disk images are generally stored in a single file and are used for duplication and backup purposes.
These files are commonly created using specialist imaging software, where each sector of the source data is read and then written to a file in a specific format. Depending on the software used to create the image, the resulting file may be encrypted (to secure the data) and/or compressed to reduce overall file-size.
A disk image is often restored using the same software that was used to create it. Many modern applications use virtual drives to access disk image data, where the disk image is mounted and then used as a normal CD/DVD drive.