What are these file types? Simply put they are all mpeg4 files, the new technology that is similar but superior to the mpeg and mp3 formats that you are familiar with. There are no rights issues (royalties) with m4a/mp4 files and the offer better compression withbetter audio quality. So ripping tracks at the same bit rate as an mp3 file would result in a smaller, better sounding track.

In a nutshell, the file formats are:

• mp4 can be an audio or video mpeg4 file
• m4a = mp4, but not necessarily vice versa. That is because m4a says, "This is an audio mpeg4 file", to you for your sake, not the computers, because the data is the same. You can safely rename m4a files as mp4, and vice versa so long as the mp4 file is an audio-only file.
• aac is an audio file, like an m4a, without the meta-data header. It's basically a nake m4a file. You can't add information into the file like Artist, Album, Track, etc. This was Apple's (rather lame, IMHO) idea.
• m4b is the mpeg4 version of an "audiobook". Haven't used one yet, but this is something I've only seen in association with iPods. I'm not sure if they're used anywhere else in general, but there is a WinAmp plug-in for this which will play the file so long as you have its license. Not sure how licensing ties into these files yet. The m4a and mp4 files I make and use don't have any such constrictions. For now, m4b goes in my suspect/evil list.

The current version of Winamp can both rip (create) and play m4a/mp4/aac files out-of-the-box, just fine. There is a plug-in for WinAmp which can be installed to play mp4 files, however I recommend to not install it unless you have a specific need for some specialized functionality it might offer. The current version of WinAmp will natively play the files and, for me, has handled them better than the plug-in. The native m4a-ripping capabilities of WinAmp are sufficient for most users as well, being easy and intuitive to configure and use.

Note: I am putting Winamp support of MP4/M4A/AAC files on the "bleeding edge" list for now. I have had multiple instances where M4A files I have ripped in WinAmp have suddenly become un-playable after I have edited the tags within WinAmp. These are both M4A files ripped in iTunes as well as in WinAmp -- it mangles them without predjudice.

I'm keeping away from M4A files for now. I'll wait for a year while other people suffer through the bugs then give it another try.