There are usually 2 or 3 pairs of VASI Boxes either side of the runway. As has been explanined, 2 rows of white lights means you are too high, 2 rows of reg lights and you are too low. How is this achieved?
Inside each VASI box there are very intense lights, and coloured filters are placed over these - top half red, bottom half white. At the other end of each box is a slit, through which you see the lights - the geometry of the boxes is determined by the glideslope angle. Now just think about looking through the slit, if you are lower down, you see the top half of the filter (red) If you are higher up you see the lower, white section.
Other approach guidance systems are;
Non-Standard VASI - French system (trust the French!)
PAPI - Precision Approach Path Indicator - Common in UK
PLASI - Pulse Light Approach Slope Indicator
CHAPI - "Chopper" system
I'm not going to explain all these because I don't have a spare 5 hours!