An is an archive file of an optical disc, a type of disk image composed of the data contetns of every written sector of an optical disc, including the optical disc file system. ISO Image files typically have a file extension of .iso, named after the ISO 9660 file system used with CD-ROM media; Note: what is known as an ISOIMAGE might also contain a UDF (ISO/IEC 13446) file system, typically used by DVDs and Blu-ray Discs.
The .iso file extension is the one most commonly used for this type of disc images. The .img extension can also be found on some ISO image files, such as in some images from Microsoft DreamSpark; however, IMG files, which also use the .img extension, tend to have slightly different contents. The .udf file extension is sometimes used to indicate that the file system inside the ISO image is actually UDF and not ISO 9660.