Ok, the terms "STEREO" and "JOINT STEREO" in Mp3's for example, refer to how the channel volumes are recorded.
It's a kind of difficult concept to explain but really all you need to know is that in Stereo audio there are two different channels being played at once, left and right and each of these channels will have different values as the sound levels fluctuate and the music is played.
In "STEREO" recording, two completely seperate values are stored and played through the two different channels, basically like this:
The "JOINT STEREO" is a much newer concept, however. It exploits the fact that a lot of the time, both channels have very similar signals and values being sent to them for playback and only records the difference from one channel as the values of the other channel.
Joint Stereo signal values are recorded something like this:
Except of course, it's much more complex than this, but this explanation was just to give you a basic idea of how the two methods differ.
Because of the way joint stereo works, it requires less bits to produce a certain quality level than standard stereo does.