This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
In common use, an "ISO" is a file that contains the complete image of a
disc. Such files are often used when transferring CD-ROM images over
the Internet. Depending on who you're talking to, "ISO" may refer to
all disc image files or only certain kinds.
Going by the more restrictive definition, an "ISO" is created by copying an
entire disc, from sector 0 to the end, into a file. Because the image file
contains "cooked" 2048-byte sectors and nothing else, it isn't possible to
store anything but a single data track in this fashion. Audio tracks,
mixed-mode discs, CD+G, multisession, and other fancy formats can't be
To work around this deficiency, software companies developed their own
formats that *could* store diverse formats. Corel developed CIF, which is
still in use by Roxio's Easy CD Creator. (What does CIF mean? Nobody
knows, though "Corel Image Format" is as good a definition as any.) Jeff
Arnold's CDRWIN created them as "BIN" files, with a separate "cue sheet"
that described the contents. You can unpack a BIN/CUE combo with
"binchunker", which is now integrated into Fireburner (section (6-1-50)).
A ".DAT" file could be most anything, but usually it's a video file pulled
off of a VideoCD. A program at http://www.vcdgear.com/ can convert .DAT
to .MPG, and recording programs like Nero can record them directly.
A ".ISO" file that contains an image of an ISO-9660 filesystem can be
manipulated in a number of ways: it can be written to a CD-ROM; mounted
as a device with the Linux "loopback" filesystem (e.g. "mount ./cdimg.iso
/mnt/test -t iso9660 -o loop"); copied to a hard drive partition and
mounted under UNIX; or viewed with WinImage (section (6-2-2)). There is no
guarantee, however, that a ".ISO" file contains ISO-9660 filesystem data.
And it is quite common to hear people refer to things as "ISO" which aren't.
A ".SUB" file appears to contain subchannel data. Some programs pass
these around in addition to one of the above formats.
We now have many different file extensions, including ISO, BIN, IMG, CIF,
FCD, NRG, GCD, PO1, C2D, CUE, CIF, CD, and GI. Smart Projects' IsoBuster,
from http://www.isobuster.com/, can open and manipulate just about any
disc image format.
(The rest of this section is a philosophical rant, and can safely be
skipped. This is intended to be more illustrative than factual, and any
relation to actual events is strictly coincidental.)
The term "ISO" is ostensibly an abbreviation of "ISO-9660 disc image",
which is itself somewhat suspect. ISO-9660 is a standard that defines the
filesystem most often used on CD-ROM. It does not define a disc image
format. "ISO-9660 filesystem image" would be more appropriate.
When you capture or generate a CD-ROM image, you have to call it
something. When a CD-ROM was generated from a collection of files into an
ISO-9660 filesystem image, it was written into a file with an extension of
".ISO". This image file could then be written to a CD-ROM. As it happens,
the generated image files were no different in structure from the images
that could be extracted from other CD-ROMs, so to keep things simple the
extracted disc images were also called ".ISO".
(Some programs used the more appropriate ".IMG", but unfortunately that was
This meant that, whether you extracted a data track from a disc written
with the HFS filesystem or the ISO-9660 filesystem, it was labeled ".ISO".
This makes as much sense as formatting a 1.4MB PC floppy for HFS, creating
an image, and calling it a "FAT12 disk image" because such floppies are
usually formatted with FAT. It didn't really matter though, because no
matter what was in the file, the software used the same procedure to write
it to CD-R.
As a result of this filename extension convention, any file that contained
a sector-by-sector CD-ROM image was referred to as an "ISO file". When CD
recorders hit The Big Time and many people started swapping image files
around, the newcomers didn't know that there was a distinction between one
type of disc image and another, and started referring to *any* sort of disc
image as an "ISO".
These days it's not altogether uncommon to see messages about "making an
ISO" of an audio CD, which makes no sense at all.
More trivia: "ISO" refers to the International Organization for
Standardization. Because the organization's name would have different
abbreviations in different languages ("IOS" in English, "OIN" in French),
they used a word derived from the Greek "isos", meaning "equal".