Common audio formats

AAC, Advanced Audio Coding File
Compressed audio file similar to a .MP3 file, but offers several performance improvements; examples include a higher coding efficiency for both stationary and transient signals, a simpler filterbank, and better handling of frequencies above 16 kHz; maintains quality nearly indistinguishable from the original audio source.
AAC is the default encoding used by Apple iTunes software.

AIF, Audio Interchange File Format
CD-quality audio file, similar to a .WAV file; developed by Apple Computer, but based on the Electronic Arts .IFF format; standard CD audio AIFF files use a sampling rate of 44.1KHz, are 16-bit, and have two channels (for stereo sound).

.MID, MIDI File
Standard MIDI (Musical Instrument Digital Interface) file; contains music data such as what notes are played, when they are played, how loud each note is played, how long each note is held, etc.; may also contain MIDI controller data for changing instruments on a MIDI hardware device, sustain pedal information, and other types of MIDI data.
Can be played back using software instruments, but does not contain actual audio data.

MIDI, MIDI File
Standard MIDI (Musical Instrument Digital Interface) file; contains music data such as what notes are played, when they are played, how long each note is held, and the loudness (velocity) of each note.
Can be played back using software instruments, but does not contain actual audio data; MIDI files commonly use the .MID file extension as well.

MP3, MP3 Audio File
Compressed audio format developed by the Moving Picture Experts Group; uses "Layer 3" audio compression; commonly used to store music files and audiobooks on a hard drive; may provide near-CD quality sound (stereo, 16-bit) in a file roughtly 1/10 the size of a .WAV or .AIF file. The quality of an MP3 file depends largely on the bit rate used for compression; common bit rates are 128 kbps, 160 kbps, and 192 kbps; higher bit rates result in higher quality files that also require more disk space.

Supported by most portable music players as well, such as the Apple iPod and Microsoft Zune; can also be played on Symbian OS mobile phones using UltraMP3.

MPA, MPEG Audio File
Compressed audio file using MPEG Layer I, II or III compression

RA , Real Audio File
Audio file created by RealPlayer; may contain audio data or may reference a streaming audio file online; uses a proprietary Real compression algorithm; common format for playing audio clips within a Web browser.

RAM, Real Audio Media
Audio file created with RealPlayer that uses the proprietary Real compression algorithm; may contain audio data or may reference a streaming audio file online.

WMA, Windows Media Audio File
Audio file compressed with Windows Media compression; proprietary format developed by Microsoft, similar to the .MP3 format; can be converted to other more standardized formats; often used for playing music from the Web.
Microsoft's Zune portable music player and the Disney Mix Stick MP3 Player can play WMA files; they cannot be played on an iPod.