Halo is a sepual, possiblely a prequal to the Marthon series that started on the Mac. What made the marthon series special was that the story and plot progression was done through a series of computers displays. Also to the best of my knowlegde, it was the first FPS to introduce Allies who fight with you and maybe also secondary funtions on weapons.
The Marthon series also introduced Rampancy, basicly computer insanity. There is a reference to it if you off the Captain on the first level of Halo.
Graphics aside, Marthon and Halo are part of the same series, but they are a world apart in style, story telling and in general atmosphere. Halo compared to Marathon was somewhat a step backwards in everything but graphics.