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 bitrates.
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.
Usually a raw AAC bitstream. Early AAC encoders often produced files like this. AAC in its newer AAC+ variant is now increasingly found on the internet as several internet based radio stations use this format to broadcast digital audio streams (raw AAC+ bitstream). To play such raw AAC/AAC+ data it is usually the best to wrap a .mp4/.m4a container around the raw AAC/AAC+ files as most media players have problems with raw AAC/AAC+ data. That can easily be done by using the program "mp4Box".
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).
In this case there is no difference between the extension (i.e. you can rename .m4a to .mp4 and vice-a-versa).
Apple's proprietary copy protected MP4 audio files. No one should be posting these on TMB under any circumstances.
The following media players will playback AAC files:
Although in theory AAC should outperform MP3 at the same bitrate, some early AAC encoders were of a poor quality and were easily beaten by LAME at medium and high bitrates. Below is a list of free encoders that produce high quality AAC files: