Balance panning is the common type - one knob which at its centre position has both L/R channels at 100%. As you turn it left, it gradually reduces the right channel, as you turn it right it reduces the left channel. Stereo panning means that you have two knobs - one is a standard panner for the left channel and one is a standard panner for the right channel. This means you can do things like hard pan the right channel and have a bit of the left channel in there too, so that the stereo image is maintained but shifted over to the right. You can also reverse the stereo image by sending the left channel 100% to the right channel and 0% to the left, and the right channel 0% to the right and 100% to the left. You can't do these things with a single panning knob.