The Forerunner is an ancient, technologically advanced and mysterious race that built the Halos and many other artifacts and structures throughout the galaxy. They died out millennia ago after a three-hundred year war with the Flood. In desperation, the Forerunners activated the Halo rings, thus killing most of their race and all potential food sources for the Flood. The Forerunners are also the ancestors of the human race, as revealed in Halo 3.
Threshold - The gas giant that Installation 04 (or Alpha Halo) orbited before its destruction in the events of Halo: Combat Evolved. There is a multiplayer map set on its moon Basis, and two campaign levels set on a gas mine in Threshold's low orbit, part of which had been converted by the Forerunners to research the Flood during the early years of their war.
Substance - The gas giant planet that Installation 05 (Delta Halo) orbits.
Halos - Seven titanic ring-shaped constructs that serve as weapons of last resort to contain the Flood. Together the Halos possess the power to destroy all sentient life in the galaxy. They are centrally controlled at the Ark, which can also rebuild damaged Halos. The fourth of seven was destroyed in Halo, rebuilt by the Ark in Halo 3, and subsequently destroyed.
The Ark - Was thought to be located in the vicinity of New Mombasa on Earth. This proves to be inaccurate as the structure buried there deep underground is actually a gateway to the Ark which resides in deep space (218 lightyears away from the Milky Way center, outside of the effective range of the Halo network and probably conventional Faster-Than-Light travel [hence the biblical reference]) and is a mechanism capable of firing the entire Halo network. Its original purpose was a storage location for all "indexed" species as well as Forerunner survivors during the firing of the Halos. The Ark is capable of ordering repairs and reconstructions of damaged HALO rings. Was destroyed by John-117, the Arbiter, Cortana and Sergeant Johnson at the end of Halo 3.
Onyx - An artificial "shield world" constructed by the Forerunners, formed from literally trillions of Sentinels interlocked into a planetary body. Served as the training ground for the SPARTAN-III program. At the core of the planet is a Slipspace rift that connects to a compressed Dyson Sphere isolated from normal space-time, which serves as a "bomb shelter" of sorts against the activation of the Halos.