M4A

MP4 or MPEG-4 part 14 is a standard based on the QuickTime format.
It is a container. The content can be any number of audio, video, and even subtitle streams.
Although the format is the same, the .M4A extension is in general used to indicate a MP4 container with audio only.

Unlike MP3, the extension says nothing about the bit rate of the content.
A M4A can contain AAC (lossy compression) or ALAC (lossless compression) or any other MPE4 audio format.

References

1. MPEG-4 File Format, Version 2 - Library of Congress