AAC
AAC (Advanced Audio Coding) is part of the MPEG-2 and MPEG-4 audio specification and is promoted as the successor to MP3 for audio coding at medium to high bit rates. It is the audio format used by the iTunes Music Store and can be played on iPods.
File Extensions
AAC has a number of common file extensions. Unlike some other media formats, there are differences between the file depending on the extension so you cannot rename from one extension to another.
.aac
Usually a raw AAC bitstream. Early AAC encoders often produced files like this.
.mp4/.m4a
AAC wrapped in an MP4 container. This is how most new AAC encoders produce files. The MP4 container can contain other audio and video formats as well as AAC, so a .mp4 file does not always indicate the presence of AAC (e.g. Apple Lossless is also put in an MP4 container).
There is no difference between the extension (i.e. you can rename .m4a to .mp4 and vice-a-versa).
.m4p
Apple's proprietary copy protected MP4 audio files. No one should be posting these on TMB under any circumstances.
Playback
The following media players will playback AAC files:
Encoding
Although AAC should in theory outperform MP3 at the same bitrate, early AAC encoders were of a poor quality and would be easily beaten by a decent modern MP3 encoder like LAME at most bitrates. Below is a list of software with high quality AAC encoders:
