ha ha ha...quite a broad question.....
people used to gauge lighting by watts per gallon, for example, a certian corals needs about 3 watts per gallon, this theory is not really used anymore as advanced lighting types become available...
I class medium lighting like VHO or power compact lighting, where as a high lighting i would class as metal halide...
There are two ways of classing lighting levels i suppose...one way as above and the other to class lighting is by how many watts you provide for the tank which, of course, is always going to be different as every tank is of a different size ( length and depth etc etc )..
Just a basic opinion of course...
Niko