A little research goes a long way. Check out www.m4a.com for the real answers. This is just a snippet from the site. It is NOT just an Apple thing.
"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."