ISO image
An ISO image (International Organization for Standardization) is an archive file (also known as a disc image) of an optical disc, composed of the data contents of every written sector of an optical disc, including the optical disc file system. ISO images can be created from optical discs or from a collection of files by image creation software; images can be used to write optical discs. Software distributed on bootable discs is often available for download in ISO image format, and used to write a CD or DVD. ISO image files often have a file extension of .iso. The name ISO is taken from the ISO 9660 file system used with CD-ROM media, but what is known as an ISO image might also contain a UDF (ISO/IEC 13346) file system or a DVD or Blu-ray Disc (BD) image.
Details
Advantages
ISO images are stored in an uncompressed format. Any CD or DVD can be archived by .ISO format as a true digital copy of the original. The ISO image file is not stored in a container file. Unlike a physical optical disc, an image can be transferred over any data link or removable storage medium.
What is known as a "valid" ISO image is an uncompressed collection of various files merged into one single resulting file, according to definite and standard formatting.
An ISO image can be rendered, or "burned," to a CD, DVD, or BD by using media authoring or disc burning software. It can also be opened using any file archiver. ISO burning is now typically a native feature of modern home and business computer operating systems.
Hybrid disc formats include the ability to be read by different devices, operating systems, or hardware. In the past, one example of this was a disc that supported both Microsoft Windows and Macintosh from a single disc image. A later example is the release of hybrid ISO files which can be booted or started from both BD or DVD and USB flash drive devices when the image is written to any of these storage devices.
An ISO can be "mounted" with suitable driver software, i.e. treated by the operating system as if it were a physical optical disc. Most Unix-based operating systems, including GNU/Linux and Mac OS X, have built-in capability to mount an ISO; in other cases software drivers can be installed to achieve the same objective.
Limitations
ISO images of CD-Audio discs cannot be made; CD-Audio discs do not use a computer file system, but are formatted in tracks; track numbers, index points and CD time code are encoded into the lead-in of the CD-Audio disc, and these reference points are found throughout the CD-Audio sub-channel. To store an accurate copy of a CD-Audio disc, a different image format must be used, such as DDP or BIN/CUE.
ISO images are also unable to store multi-track disc images.
As of February 2011 no "portable" programs for the Windows OS were known to exist that were capable of mounting ISOs on a USB flash ("thumb") drive or of reading them from such a drive. It is believed that this was because mounting and/or reading ISOs in Windows NT and later, network-based, versions of Windows OS required a driver, which is inaccessible to users of these operating systems who lack administrative privileges.