Joint stereo is a bit less accurate then stereo. With stereo, each channel allocates half of the bitstream. Joint-stereo takes advantage of the fact that theres usually only small differences between the two channels, thus encodes only the common signal and the different signal and allows the player to basically re-assemble them.
In other words, if both L/R channels are identicle, it only stores one channel, then grabs whatever piece of what channel is different, ignoring the rest because it's the same as the other channel.
from another site mate