I'm not 100% sure, but my guess is that they are just .M4a files but renamed as .m4r. But what I understand is that Apple Ringtones are time limited (I mean not able to be a complete song, but just an excerpt of that song).
I was just curious... Because last time I synced a tone I downloaded it appeared on my device with some weird chinese characters.. but the file name was (for example) "Ringtone_2.m4r"
So somewhere in the file must be additional tag/information
Im creating Lots of ringtones from (mostly from Instrumentals) and I don't want to be guessing which ring tone was which when its time to select a ringtone.