I'm glad that you asked. Ogg Vorbis is a format for music files like MP3. In many ways, I think that Ogg Vorbis is better. It produces higher quality output at a lower bit rate than MP3's. This means Ogg Vorbis files sound better and are smaller than MP3 files. In addition, Ogg Vobis is an open format and it uses open source software. This means that if you wanted to create an Ogg Vorbis player, or write a program that uses Ogg Vorbis, you don't need to pay any licensing fees on the technology. The MP3 format is patented by a German company (Fraunhofer Society) and they charge licensing fees to use it.
So, why isn't it more popular you may ask? I think it all comes down to timing. Ogg Vorbis was introduced much later than MP3, and several MP3 codecs have been released as free software for individual use. I'm hoping that Ogg Vorbis will gain momentum and eventually win out.
If you are interested in free and open audio compression you might also want to check out the Free Lossless Audio Codec (FLAC). The is no degradation to the quality of the sound and it is free and open like Ogg Vorbis.