A single electronic file that contains the identical content of an optical disc, including the folder and file hierarchy. Using an .ISO file extension, ISO images are created to distribute the disc's data over a network in order to burn a CD or DVD at the destination computer. The files within the ISO image are not compressed; however, an ISO utility program is required to identify them and burn the disc with the original file/folder structure.Bootable Discs and USB DrivesISO images are the foundation of a bootable CD, DVD or USB drive. However, the boot program must be added by using a utility program. For example, WinISO makes CDs and DVDs bootable from ISO images, while Rufus does the same for USB drives. See Rufus, ISO 9660, UDF, DMG and disk image.