The difference is slight. This is what Wikipedia has to say:
A total of 50 pre-series and series D.I aircraft were in service by November 1916, replacing the early Fokker and Halberstadt D types, giving real "teeth" to the Luftstreitkräfte's new Jagdstaffeln (fighter squadrons). Further production of D.Is was not undertaken, however; instead, a reduction in the gap between the top and bottom planes in order to improve the pilot's forward and upward vision resulted in the otherwise identical Albatros D.II,[1] which became Albatros' first major production fighter.
cheers
David