There's more to it than this but very simplified:
A fearful dog is reacting to a non threatening situation or thing.
A protective dog reacts to an actual threat. (this can include a perceived threat by you if you are actually scared).
Raven (GSD) - December 8, 2007
Kaiser (GSD) - November 2009
Holly (GSD) - March 24, 2011
Best Paw Forward