As mentioned above, there are several different audio formats which are essentially extended forms of AAC.

• MP4 - The standard container format for MPEG-4 media. MP4 may contain both audio and video. MP4 files may be played either QMP's AAC plugin, or by adding MP4 to QMP's list of video file types and installing the appropriate DirectShow filter for Windows.
• M4A - This extension is used primarily by Apple for MP4 files that contain only audio, not video.
• M4P - An M4A file that has been encrypted with Apple's FairPlay DRM. These files normally come from the iTunes store. QMP cannot play files locked down by FairPlay, however some users have reported successful playback of these files by first stripping the DRM from their files using a tool like QTFairUse6 or myFairTunes. As of this writing, they can only be used with iTunes 7.5 or earlier, newer versions won't work. (For more info, or if those links go down, you can always Google it…)

Note: To play M4A and M4P files in QMP, you may also need to change the file extension to .mp4

• 3GP - A file format primarily used by 3G phones for storing/playing audio and video. This format has not been tested with QMP.
• HE-AAC - High-Efficiency AAC (also known as AAC+ or aacPlus) is AAC with the addition of Spectral Band Replication (SBR). SBR offers significant sound quality benefits at lower bitrates (below 80kbps), but makes no difference at medium to high bitrates. A player which supports AAC but not SBR will play an HE-AAC file as though it were a standard AAC file of the same bitrate.
• AAC-LC - 'Low Complexity' AAC is the most common form of AAC. There is also the 'Main' profile of AAC, however it requires considerably more processing power to decode, and is not widely used.

Recent versions of Shaohao's MP4 plugin support streaming AAC.

(section unwritten)

http://wiki.hydrogenaudio.org/index.php?title=AAC
http://en.wikipedia.org/wiki/Advanced_Audio_Coding