The Halo trilogy is a series of games created by Bungie, and owned & published by Microsoft Game Studios.  The story revolves around a cybernectically-enhanced human Master Chief Petty Officer John-117, and his Artificial intelligence named Cortana. The Master Chief is put up against an invading alien force known as the Covenant.  In the series, Halo is referred to as the large ring-shaped structures that allow biological life to live on them.  These structures are massive and are sometimes as large as a planet; however, it is still a ring, and as such, has nothing in the middle.  The ring spins to create a gravitational force.

Master Chief                                                Cortana