Here
Short answer: M4a is the succession of the mp3 format. This may have drm. (wikipedia says: ".m4a is now being used by the iTunes Store for DRM-free, unprotected music files." so probabably your ipod should read that file)
Have you heard about
ogg? IMHO is the best audio container out there!

-> The ipod doesn't read this kind of file natively. You can find some hacks in google.
Cheers