What are the Cortana Letters? Beginning in February 1999, the Marathon's Story page received a series of cryptic letters from an anonymous source at Bungie Software. Eventually they were traced to Nathan Bitner, who continues to deny he was responsible, and who was later revealed as Halo's producer and creative director . These letters are (mostly) written from the perspective of an entity named Cortana, who seemed to be an artificial intelligence fighting a race of aliens. For (much) more information on the Cortana Letters consult the Marathon's Story page. The Cortana Letters have been analyzed by Bungie fans many times over, some of their conclusions can be found in the Forum archives and on the Theories page.
What are the halo.bungie.com Transmissions? When Halo was revealed at MWNY, its official site at halo.bungie.com went live. Among its content was a collection of short messages, which it would display randomly each time the page was loaded. These were in the form of computer transmissions and included other bits of information from which many conclusions have been drawn regarding the Halo universe. They were removed when the site was redesigned in October 1999, but they have been archived on the Marathon's Story page. For more information check the Forum archives and the Theories page.
There are 4 major bodies in the Halo system: The sun, the gas giant Threshold, its moon Basis, and the Halo. Threshold orbits the sun. Basis orbits Threshold. The Halo stays at a Lagrange point of Basis and Threshold in a halo orbit, it is always in the same place relative to them.
The Halo: The Halo is an enormous ring, 10,000 kilometers across. Its width and the thickness of the "floor" is unknown. the Halo's contents are held to the inner surface of the ring; it is rotating in place to create centripetal gravity. The inner surface of the Halo is covered with ground, complete with hills, mountains, rivers, and oceans. Scattered around the Halo are various buildings. The Halo has an atmosphere, that is apparently held in at the sides by a set of very high walls, much like the Ringworld. As the Halo's terrain is on the inner surface of a ring, the Halo itself can be seen curving into the sky as you walk outdoors, and is visible in many of the screenshots.
What does Marathon have to do with Halo? From the beginning of Halo's development and marketing, it appeared that Halo was related to Bungie's previous game Marathon. The Cortana Letters were sent to the Story Page, and early Halo material had some Marathon-like aspects. However, with the release of the final game and its final plot, it has become clear that the Marathon connections are, at best, extremely tenuous. All Marathon-related items in Halo are found in easter eggs, in-jokes, sly references, and other places that do not support a real storyline connection.
What is Halo: The Fall of Reach? Halo: The Fall of Reach is the official tie-in novel to Halo, written by Eric Nylund with the Bungie team's assistance. It is a prequel to the game, exploring the origins of the game's major characters, the war with the Covenant, and the events leading up to the discovery of Halo by the Pillar of Autumn. It contains a great deal of story-related information and explains a good deal of the passing references in the game, as well as fleshing out the Halo universe and in the opinion of yours truly is some rather good military science fiction.
What is the Halo Story Bible? The existence of a Halo Story Bible was revealed during discussion surrounding Halo: The Fall of Reach. It is Bungie's compendium of all things Halo, the canonical guide to the universe. However, Bungie has stated that they will not release this bible to anyone unofficial (not even to writers of fan fiction).