If I remember correctly, iTunes songs are m4p, Apple-DRM'ed AAC encoded files. Once the DRM stuff is removed, they are normally m4a, AAC encoded files in the MPEG 4 (Audio) containers.
These days we now have to deal with both the encoded data and the container. (Which is actually better than the old days, which leads to the whole mp3 tagging problem.) The encoded data uses a specified codec. The container includes the compressed data and meta information like tags. Some codecs include a mechanism for meta information like FLAC, others don't like mp3.
Ogg, for example is actually a container, and Vorbis is the codec, hence mentions of Ogg Vorbis. The original vision for the Ogg container format was much grander, with Theora as a video codec as well as alernate audio codecs like Speex and FLAC. Occasionally you run across mention of Ogg FLAC which is using the FLAC codec inside of an Ogg container, which isn't necessary but might be useful in some circumstances.
Windows has a real problem with the same extention being used for multiple kinds of data, and the slow progress (and the Ogg project not providing any practical means to have the operating system deal with different types of payloads) and basically lack of adoption meant that ".ogg" was generally assumed to mean only Vorbis payloads.
--Nathan
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