Ogg Vorbis is not MP3!

There are only two types of MP3. The original MP3 format which is also known as MPEG Audio Layer 3 and an enchanted - but not very successful - version called MP3 Pro (the latter is not supported by iTunes).

Ogg Vorbis:

http://www.vorbis.com
http://www.vorbis.com/faq.psp
http://en.wikipedia.org/wiki/Ogg_Vorbis

MP3:

http://en.wikipedia.org/wiki/MP3 (see Licensing and patent issues)
http://www.iis.fraunhofer.de/amm/techinf/layer3

MP3 Pro:

http://en.wikipedia.org/wiki/MP3Pro