Jedi Dark, Jedi Bright

Just what is it that distinguishes a light side Jedi from a dark side Jedi and vice-versa? Is it what lies on the surface or beneath it? *** This story is set in 616 BBY, long after the New Sith Wars and long before the Clone Wars. As it is set in my own personal canon, there are going to be some slight inconsistencies with established film/EU continuity.