M4A file format

The m4a file format is the file format for a file that contains MPEG-4 Audio. MPEG-4 Audio is the successor of the popular MP3 standard for audio coding. MPEG-4 Audio is based on the MPEG-4 Advanced Audio Coding (AAC) standard for the compression of audio. MPEG-4 Audio has a better quality than MP3 at the same bit rate. Or to put it the other way around, the bit rate can be less, and the file smaller, for the same quality.

What is the difference between a .m4a and an .aac file?
A m4a audio file is based on the mp4 file format. Some programs use the generic extension mp4 for mpeg-4 audio. A MP4 file is a container that can contain about anything, video, audio, still pictures, text etc. The extension m4a says that the file does not contain any video, but only audio. the file can also contain related infomration about the artist, the song, the cd etc.
A file with the extension aac only contains the audio. It is not put in a mp4 container and does not contain any associated information. Some audio player can play m4a audio files but can not handle aac files.

Most players nowadays can handle m4a and mp4 audio files. If you have an old player, e.g. an older version of Winamp, you can look at RareWares for a plugin.