In the most obvious sense, the "Halo" is a space station built in the shape of an enormous rotating wheel. As the wheel spins, it creates centrifugal force that mimics gravity on its inner rim. An entire world exists inside this wheel, with huge developed structures and vast, open landscapes. But something else exists in the Halo as well — something mysterious and powerful. All that is really known is that a force of alien invaders is desperate to find it and they are leaving death and destruction in the wake of their search. This mystery of Halo must be solved and the aliens must be vanquished, before they find what they are looking for and destroy us all.

The action in Halo is squad-based. While gamers have lots of options when making decisions about the individual characters they will play, many missions are best accomplished through teamwork and certain multiplayer capabilities are supported with this in mind. The 3D engine's default view switches between first-person and third person, depending on the situation. Much of the action takes place in the wide open spaces inside the mammoth space station, with no level breaks or load times as the player moves from one place to another.

Some of the basic themes and plot points in Halo may be familiar to gamers who have played Marathon, Bungie's successful first-person shooter which was released in 1992 for the Macintosh platform. Among other elements, Marathon is known for intelligent level design and a deep, intriguing story. Though Halo should not be considered a sequel to Marathon, the two share common plot elements and have related back-stories. Initial development of Halo was done simultaneously for the Macintosh, the PC, and the Xbox. ~ T.J. Deci, All Game Guide