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An ISO image is a type of disc image that acts as an archive file that is comprised of all sector data contained in an optical disc, including its file system. Image files have a file extension of .iso, which is taken from the ISO 9660 file system used in CD-ROM media. However, ISO images may also contain a Universal Disk Format (UDF) file system, which is used in DVDs and Blu-ray discs.
An ISO image is also known as an ISO file.
An ISO image contains exact copies of an optical media file system with the data in binary format and copied exactly as it was stored on the disc. The data within an ISO image is ordered according to the file system used on the optical disc from which it was created. ISO Images only store data, disregarding control headers and correction data, hence they become smaller than the raw data on the optical media.
The .iso file extension is the most commonly used, but the .img file extension can also be found on some ISO image files. The .udf file extension is sometimes used to indicate that the file system inside the ISO image is actually UDF and not ISO 9660. There is no single standard format, so the term “ISO image” is used in a broader sense to refer to any disc image file of an optical disc, independent of the format it uses.
A common use for an ISO image is for temporary storage before it is written to a blank CD-R or DVD-R, creating an identical copy of the original disc. ISO image files may be opened and their contents copied to a local folder. They may also be virtually mounted and accessed as a CD drive. They are often used to distribute large programs over the Internet due to the fact that all of the program’s files can be neatly enclosed as a single file.