I'd recommend you this article
about mp3 bitrate, which is similar to the one used in ogg vorbis
The simple mechanics of these codecs are to remove frequencies of a sound that a standard human ear can't react to. That's why they are lossy, that's why they might leave artifacts, that's why they weight a lot less than the original recorded sound.
These bitrates are the standard settings for encoding bitrates, meaning they are set just so and even if the final file is lossy, there's still room for a little more "cutting".
That's why there are constant and variable encoding bitrates (respectively CBR and VBR).
On VBR, the cutting process makes the bitrate variable so that you save even more space.
I generally use oggdrop from the vorbis organisation to encode in ogg. It's really simple: drag& drop and your file is being encoded whether on the selected bitrate if you encode in CBR or directly in VBR.
You can find this wonderful tool there
I generally encode in VBR to save some space, you don't really notice any quality difference.
Well, if you have an absolute pitch
ability, please pardon me if I inflicted you such stress.