One of the facts which the public is well-aware of, is that Sound and Video are usually distributed in compressed form, through the use of a ‘Codec’, which stands for ‘Compressor / Decompressor’. What may still have some people confused though, is that there is a separate distinction in file-formats, which is the ‘Container File Format‘. The latter distinction is observed, when giving the file its filename-suffix, such as .MP3, .MPEG, .MP4, .OGG, .M4A, etc..
- An .MP3-File will contain sound, compressed with the Codec: MPEG-2, Layer III
- An .MPEG-File will contain video and sound, compressed with the Codecs: MPEG-2 or MPEG-1, And AC3 or MPEG, Layer III Audio (Hence, ‘MP3 Audio’ is allowed.)
- An .MP4-File will contain video and sound, compressed with the Codecs: H.264 or MPEG-4, And AAC
- An .OGG-File will mostly contain video and / or sound, compressed with the Codecs: Theora (video) And Vorbis (sound)
Finally, because the ‘AAC’ Sound Codec, which stands for ‘Advanced Audio Codec’, has qualities which have been found desirable outside its initial usage-scenario, for movie-making, just for Audio, there has been some possible confusion, as to how the users should name a container file, which contains only AAC-compressed audio, but no video. On my Linux-computers, I’m used to giving those files the filename-suffix ‘.M4A’ . Other people may at one time have been doing the same thing. But because the suffix was not widely recognized, Apple specifically, may have just started the trend, of just naming the container files ‘.MP4-Files’ again, even though they contain no video. This may simply have helped their customers understand the file-formats better.
The AC3 and AAC sound Codecs both offer directionality in the sound, which was useful for movies, but which will exceed the degree of directionality, that ‘MP3 Audio’ offers. And so, even though AAC offers small file-sizes, it has become popular for Music as well, because the way in which the Advanced Audio Codec compresses its sound is ‘so smart’, that listeners tend to hear very high-quality sound anyway.