Successor to mp3, m4a (M4A) is the new replacement for the older MP3 audio format and includes many enhancement and improvements.
MPEG Layer 4 audio. A fileformat generally used with music files that is higher in quality for about the same filesize.
Files ending in .m4a are audio content only, .mp4 can contain both audio and video, .m4v are video files.
Within AAC are different encoding types, LC (low complexity), HE (high efficiency) to name a few. To encode your audio to be compatible with most audio players, use the LC compression type (used as default by this encoder). High Efficiency (HE AAC) uses SBR technology (not too different as mp3PRO) if the player knows about HE the extra frequency information can be decoded, otherwise half the audio quality will be missing on playback.
A newer standard again for ultra low bitrates is AAC+ or AAC HEv2, this uses parametric stereo for even lower bitrates, dBpowerAMP is able to decode every AAC type (excluding iTunes m4p protected).
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Apple supplies protected mp4 files in a .m4p format from its online audio store (whilst dBpowerAMP cannot decode these see Forum for details on fair use). Files ending in .aac are raw audio files.
Many popular applications and hardware devices already support M4A. The m4a
Many people have questions about M4A, since it is relatively new. M4A stands for MPEG 4 Audio, and it is a popular file extension used to represent audio files.
Most people are familiar with MP3 and how it shrinks down the file size of songs and other audio files. M4A and MP4 do the same thing as MP3 does, but even better.
Quality is better and file sizes are usually smaller than MP3 files.
But unlike MP3, no licenses or payments are required to be able to stream or distribute content in M4A format (unlike MP3 which requires you to pay royalties on content you distribute in MP3 format).
This fact alone, is more than enough reason (due to the extreme cost savings) to use M4A files instead of MP3 files.
In addition, M4A files tend to sound much better than MP3 files encoded at the same bitrate.
What does M4A stand for?
Many people have questions about M4A, since it is relatively new. M4A stands for MPEG 4 Audio, and it is a popular file extension used to represent audio files. Most people are familiar with MP3 and how it shrinks down the file size of songs and other audio files. M4A and MP4 do the same thing as MP3 does, but even better. Quality is better and file sizes are usually smaller than MP3 files. But unlike MP3, no licenses or payments are required to be able to stream or distribute content in M4A format (unlike MP3 which requires you to pay royalties on content you distribute in MP3 format). This fact alone, is more than enough reason (due to the extreme cost savings) to use M4A files instead of MP3 files. In addition, M4A files tend to sound much better than MP3 files encoded at the same bitrate.
What is the difference between M4A and MP4 files? I am a bit confused.
This is the most popular question we get and seems to cause the most confusion to people new to MPEG 4 Audio. The existance of 2 different file extensions that can be used to represent MPEG 4 Audio files is unfortunate. MP4 files may or may not contain MPEG 4 Audio. If you see a M4A file you always know that it contains only MPEG 4 Audio<. MP4 can be used for MPEG 4 video files, combined video and audio files, or just plain MPEG 4 audio. Apple Computer started using and popularizing the M4A file extension to denote the file was an unprotected (non digital rights management) MPEG 4 Audio file. They did this because MP4 was too general (video, video/audio or audio) and might confuse some media players. Now MPEG 4 Audio has its own file extensions, M4A, to avoid any possible problems with being confused with video files. It is recommended that you use the .m4a file extension rather than .mp4 on your audio files. Up until recently, there was much confusion among MP4/M4A encoder and player software. Some programs (Nero, Compaact) used .mp4 while others (WinAmp 5.02, Apple iTunes, iPod) used .m4a to denote MPEG 4 Audio files. Most software developers have now enabled a user selectable option in their software to allow you to choose the default file extension you wish to use to save MPEG 4 Audio files with. Almost all audio players will now play back files using either the .m4a or .mp4 file extension for maximum compatibility. After all, both the .m4a and .mp4 container file formats are the same, they just have different file extensions. If your software program doesn't recognize your file extension, you can rename the file extension to the other one (i.e. m4a or mp4) and it should work.
What are .AAC files? Should I save files with the .AAC file extension or .M4A?
AAC stands for Advanced Audio Coding and is the "backbone" behind both the MPEG 2 and MPEG 4 audio file formats. MPEG 4 Audio files contain (have encapsulated in them) AAC audio streams. Basically .AAC files are audio files that are not contained in a MPEG 4 Audio container file. They are the raw "building blocks" that are used to make up M4A/MP4 files. The use or distribution to others of .AAC files is not recommended as there is not a "tagging standard" for them. Based on the opinions of audio professionals we have received, you should always save your MPEG 4 Audio files in a container file and use either the .m4a or .mp4 file extension. Plain audio .AAC files are not designed to contain song/album information like .m4a files can. Many players (such as Nero, Apple iTunes, iPods) will refuse to play .AAC files. To avoid these problems, always save your audio files to .m4a or .mp4 format instead.
Transcoding?
How best to convert audio files from MP3 to M4A/MP4 format?
Don't do it using MP3s! Transcoding is the process of converting audio files from one format to another. Since many people have their audio files stored as MP3 files, this is a very common question. MP3 is a lossy compression codec (algorithm) that "tosses away" some of the original audio information when a MP3 file is created (encoded). MPEG 4 Audio is a much better encoder than MP3 is (quality-wise and resulting file size-wise). The problem is that going from one lossy audio format to another introduces even more loss in the conversion process (transcoding process). It is the advice of audio experts to never transcode from MP3 to M4A. The loss and distortion is not recommended. Instead, use your .WAV master audio files, re-rip the master audios from your audio CDs or use your audio files that are stored in a lossless audio file format such as FLAC or APE. There are many programs available that can convert from a lossless audio file (or audio CD) to a pristine sounding MPEG 4 Audio file (M4A or MP4). See the list below to locate many free and commercial M4A/MP4 encoding applications.
What About Microsoft's WMA and WMAPRO formats? What are they and how do they compare to M4A/MP4 audio?
I don't recommend using WMA. Microsoft has for years promoted their proprietary (i.e. closed) audio formats and codecs. They routinely have changed their encoding routines, sometimes breaking backward player compatibility on some platforms (i.e. when audio files encoded in newer WMA or WMAPRO formats can't be played back on previous/older versions of Windows Media Player or other audio players using older versions of Microsoft's WMA libraries). Using a proprietary standard like WMA that is owned, dictated and maintained by one company (in this case Microsoft) is not a good idea in my opinion. MPEG 4 Audio on the other hand uses an Internationally approved MPEG/ISO standard. This MPEG 4 Audio standard exists to enable your audio files to play properly on multiple platforms and on various hardware and software players for years to come. WMA can't make that statement due to its closed nature.
The next step up in quality after 320kbps MP3/WMA is WAV goodness. 
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