Great site kita, I have saved it.
As a short answer, the black and tan has solid colouration of black bits and tan bits (or black and gold) whereas the sable colouration is single hairs carrying different colours i.e. lighter and darker colours on one strand of hair.
Most sables are either wolf grey colour, or gold sable or silver sable, and although as kita's link shows there are other sables, these are the most common.
Have to say that gold sable is my favourite colour for GSD's.
When judging colour should not come into it unless the dog is an undesirable colour according to the breed standard, although the UK breed standard does not have any actual disqualifying faults.