A disk image is a software copy of a physical disk. It saves the entire data from the disk, including the file structure and all files and folders from the disk, in a single file. Because disk images are exact copies, or "clones," of original disks, they can be used to duplicate disks or serve as full backups in case a system restore must be done.
Disk images can be created from both hard disks and optical media, such as CDs and DVDs. However, optical media images are technically called "disc images" instead of "disk images." Several programs, such as Nero, IsoBuster, and Norton Ghost can be used to make disk images for Windows. Programs like Apple Disk Utility and Roxio Toast can create disk images for Mac OS X.
Most disk image files store data in a raw, binary format. This means they do not have a file system, which tells the computer how to access the files and folders in the disk image. Therefore, in order for the data in a disk image to be readable by the computer, the image must first be mounted by either the operating system or a disk utility program.