While Ogg Vorbis is a completely free, open source, and unpatented format, Mp3 isn't.
They are both lossy formats but Mp3 is by far the most popular one, though being extremely popular doesn't mean having the best quality.
Ogg Vorbis may very well be the future of lossy audio formats while Mp3 will slowly fade away and eventually become users' second choice.
Or perhaps lossless audio formats like FLAC, Monkey's Audio, LPAC or WavPack are the future of audio compression... ;)