The large amount of file formats on the internet today has given rise to a lot of confusion. There are some audio files distributed at .mp4 making you think they are Mpeg 4 video files. This is a list of the most popular used, and ones that will play in the default Windows Media Player, so long as you have the supporting codec's.

Formats supported by Windows Media Player

• Windows Media Formats (.asf, .wma, .wmv, .wm)
  Advanced Systems Format (.asf)
  Windows Media Audio (.wma)
  Windows Media Video (.wmv, .wm)

• Windows Media metafiles (.asx, .wax, .wvx, .wmx, .wpl)
  Advanced Stream Redirector (.asx)
  Windows Media Audio Redirector (.wax)
  Windows Media Video Redirector (.wvx)
  Windows Media Redirector (.wmx)
  Windows Media Player Playlist (.wpl)
  Digital Video Recording (.dvr-ms)

• Windows Media Download Package (.wmd)

• Audio Visual Interleave (.avi)

• Moving Pictures Experts Group (.mpg, .mpeg, .m1v, .mp2, .mp3, .mpa, .mpe, .mpv2, .m3u)
  MPEG-1 (.mpeg, .mpg, .m1v)
  MPEG Audio Layer III (.mp3)
  MPEG Audio Layer II (.mp2, .mpa)
  M3U (.m3u)

• Musical Instrument Digital Interface (.mid, .midi, .rmi)

• Audio Interchange File Format ( .aif, .aifc, .aiff)

• Sun Microsystems and NeXT (.au, .snd )

• Microfine (.mf, .mfv, mfa)

• Audio for Windows (.wav)

• Indeo Video Technology (.ivf)

• QuickTime Content (.mov, .qt)

• CD Audio Track (.cda)

Formats NOT supported by Windows Media Player

• RealNetworks Content (.ra, .rm, .ram)

• QuickTime Content (.avi, .mov, .qt)

• MPEG-4 (.mp4)

When ever you have a problem trying to playback a media file, you need to identifier its type, what codec's are needed, and if the file is genuine and not one renamed. Start by playing a file of the same extension, if that plays without problems, this could mean the file is not what it says it is.

There are many tools around the internet that can be used to identifier media files, and what (If anything) is being used to play the file. One of the most simplest methods to check an AVI is to right click the file and select properties, This will show the default properties of the video and audio, while programs like Gspot will show a lot more information.

If the media file contains compressed video, the program should show what is used to decompress it. The decompressor is normally referred to as a codec

If the right codec is installed, it will be used to decompress the file, once decompressed it can be played back. In some cases Windows Media Player will try to download the codec if its not installed on your system, however it will only do this for its own codec's, and not other peoples.

Formats, and what will play them

ASF, WM, WMA and WMV

• ASF
  The ASF format (Advanced Systems Format ) is the default format for Windows Media Player V7 if you have the correct codec installed these will playback on most systems. ASF media files that have audio compressed with WMA and video compressed with WMV use the WMV extension. While if the ASF contains media compressed by something else, the ASF is used.

• WMA
  The WMA format (Windows Media Audio) are audio only files

• WMV, WM
  The WMV (Windows Media Video) are media files with audio or video or even both, having WMA and WMV

• ASX, WAX, WVX and WPL
  ASX files are just text files, these are used to provide information about a stream presentation, used to give locations of the real file etc

• WMM
  These are XML files again used to set up the player

• WAX
  The WAX (Windows Media Audio Redirector) are used to reference WMA files

• WVX
  The WVX (Windows Media Video Redirector) are used to reference WMV files

• WMX
  The WMX (Windows Media Redirector) are used to reference WMA or WMV files

• WPL
  The WPL (Windows Media Player Playlist) is used by Windows Media Player 9 to create dynamic play lists.

• DVR-MS
  The DVR-MS (Microsoft Digital Video Recording) Video in this format is Mpeg 2 while audio is Mpeg 1 Layer 2, used mostly for TV content. These media files can be protected, to play back the unprotected DVR media files you should make sure you have Windows XP and its latest service pack.

• WMD
  The WMD (Windows Media Download) Are a package of skin boarders, play lists, media all in one file. These files can also contain advertisements and links

• AVI
  The AVI (Audio Visual Interleave) media file is the most common format to be found on the internet, its audio can be compressed with just about anything from OGG to MP3 while its video can be compressed with codec's like DivX, Xvid. Uncompressed AVI's are quite large, using compression codec's can shrink them down to move than half the size.

• Codec's used in AVI
  DivX http://www.divx.com
  Cinepak  http://www.cinepak.com
  Indeo codec http://www.ligos.com 
  DV , MJPEG, MP3 etc...

• Moving Pictures Experts Group
  (.mpg, .mpeg, .m1v, .mp2, .mp3, .mpa, .mpe, .mpv2, .m3u)
  MPG, The most common format being Mpeg 1, mostly used for Video CD
  M1v, are usually Mpeg 1 streams that contain only video information, unfortunately, .mpg do not exclusively use the mpeg 1 video streams, mpeg 2 also use the .mpg extension. You will need to install a mpeg 2 media filter, as the default Windows Media Player wont play them, The Ligos filter is the most popular, and once than is supported in programs like TMPGEnc for encoding from Mpeg 2 to something else

• MP3
  This media format is the most popular audio format around, developed by The Fraunhofer Institute in Germany in 1991, to be able to distribute MP3 files, you need a licence from Fraunhofer . However the new OGG Vorbis audio codec is gaining popularity, especially as its open source and free to use.

• MP2, MPA
  MPEG Audio Layer II is an audio-coding standard that was originally developed as a part of the MPEG-1 specification and was later updated for the MPEG-2 specification.

• M3U (.m3u)
  The MPU file is a play list that references .mp3 files

• MID, MIDI, RMI
  MIDI (Musical Instrument Digital Interface) often using .mid for the extension is a information file to exchange musical information for instruments, synthesizers, and computers these files contain information such as a start of a note, its pitch, length, volume, and musical attributes

• AIF, AIFC and AIFF
  Audio files developed by Apple

• AU (Unix Audio) and SND (Sound)
  Audio files developed by Sun Microsystems

• WAV (Wave Form Audio)
  Uses Pulse Code Modulation, Similar to the way an AVI works, WAV is just a container that can store different sampling frequencies, mono or stereo. Mostly used with ADPCM (Adaptive Differential Pulse Code) or the uncompressed PCM (Pulse Code Modulation)

• CDA (CD Audio)
  tracks are audio files that are stored on CD media, a file representation of an CD audio track, there are programs around to let you see and copy the files from a CD

• IVF
  Indeo Video Files http://www.ligos.com
  IVF files are video files that are encoded by using the Indeo codec from Ligos

• WMZ and WMS
  Windows Media Player Skins, WMS is an XML text document, while WMZ is a compressed Zip file

• MOV or QT
  Quick Time Content , developed by Apple Computer. QuickTime format can contain video, animation, graphics, 3D and virtual reality (VR) content. Only older files can be played with Windows Media Player, you will need the Quick Time Player for V2 files http://www.apple.com/quicktime

• RA, RM and RAM
  Real Networks, you need the Real One player to play these files http://www.real.com

• MPEG-4 .mp4
  MPEG-4 is an International Standards Organization, The standard covers many aspects of multimedia presentation including compression, authoring and delivery. You can implement only portions of the MPEG-4 specification and remain compliant with the standard. The MPEG-4 file format, as defined by the MPEG-4 specification, contains MPEG-4 encoded video and Advanced Audio Coding (AAC)-encoded audio content. It typically uses the .mp4 extension. You can play back .mp4 media files in Windows Media Player when you install DirectShow-compatible MPEG-4 decoder packs. DirectShow-compatible MPEG-4 decoder packs include the Ligos LSX-MPEG Player and the EnvivioTV http://www.ligos.com/lsx_mpeg_player.htm  http://www.envivio.com/products/etv/download.jsp

• Microsoft uses the Microsoft MPEG-4 v1,  Microsoft MPEG-4 v2 , Microsoft MPEG-4 v3
  MPEG-4 video content can be encoded and stored in an .asf file container by using Windows Media Tools and Windows Media Encoder, Microsoft MPEG-4 v3 , it is this that is sometimes found in AVI files around the internet made with older versions of the codec, unlocked to AVI
  http://www.microsoft.com/windows/windowsmedia/WM7/mpeg4.aspx