An ISO image is an archive file (a.k.a. disk image) of an optical disc using a conventional ISO (International Organization for Standardization) format that is supported by many software vendors. ISO image is an unattached file format (like RAR and ZIP): it merges a series of files into one single file according to a definite format. ISO image files typically have a file extension of .ISO but Mac OS X ISO images often have the extension .CDR.