It is now common, especially when working on Windows PCs or Macs, to work with
Atari software as files or "images" containing the data from an entire disk,
data cassette, or cartridge as duplicated from the native media for the Atari.

Here is a list of file formats, arranged by their associated filename
extensions.  These are all filename extensions used to name files containing
entire 8-bit Atari floppy disk images, cassette tape images, or cartridge
images.

.ATR -Image format invented by Nick Kennedy, for his SIO2PC project.
      Very similar to .XFD but with an added 16 byte header.
      This is the most common image format, used with most 8-bit Atari
      emulators running on other computer platforms.
      SIO2PC is at http://pages.suddenlink.net/wa5bdu/sio2pc.htm

.ATX -Image format invented by Jorge Cwik, for VAPI project.  Goal of Vapi is
      the preservation of Atari software in its original unmodified form,
      including custom format or copy protection.  http://vapi.fxatari.com/

.CAS -Cassette image format invented by Ernest R. Schreurs, for his 
      Digital Cassette Image system (includes CAS2SIO, WAV2CAS, and CAS2WAV
      MS-DOS utilities.  See: http://home.planet.nl/~ernest/
      
.DCM -Image format invented by Bob Puff for his Disk Communicator 3.2 utility.
      Used when working with native Atari hardware.  A compressed data format.
      DISKCOMM is at http://www.nleaudio.com/css/files/DISKCOM.ARC
      .DCM specs at: http://home.planet.nl/~ernest/diskcomm.zip

.DD  -Early filename extension used with double density disk images for use
      with the Xformer emulators.  Replaced by the .XFD extension.

.DI  -Image format invented by Kolja Koischwitz & Christian Kruger for their
      800XL DJ emulator for the Atari ST.

.PRO -Proprietary image format invented by Steven Tucker, for his
      APE ProSystem device.  Used with APE, the Atari Peripheral Emulator.
      APE and APE ProSystem are at http://www.atarimax.com/

.SCP -SpartaDOS SCOPY image file.  SCOPY was a utility by ICD.  A compressed
      data format.

.SD  -Early filename extension used with single density disk images for use
      with the Xformer emulators.  Replaced by the .XFD extension.

.XFD -"Xformer Floppy Disk" image format invented by Emulators, Inc. (Darek
      Mihocka) for the Xformer emulators (ST, PC).  Known earlier, before
      support for arbitrary disk sizes was added, as .SD or .DD depending on
      the density of the imaged disk.  The format consists simply of a raw
      sector dump of a disk.  Used with ST Xformer, PC Xformer, and 
      Xformer 2000 emulators.
      Xformer emulators are at http://www.emulators.com/
      
See also:
Atari Disk Image FAQ  (Steve Tucker)
http://www.atarimax.com/ape/docs/DiskImageFAQ/