The Halo Array, Halos (as they are referred to by the Covenant), alternatively known as the Sacred Rings by the Covenant, Fortress Worlds by their creators, and Installations by the AI Monitors that run them, are seven massive ring-shaped installations constructed by the Forerunner throughout the galaxy over one hundred thousand years ago as a last resort when combating the parasite known as the Flood. When nothing else could be done, they activated the rings which killed all sentient life forms within three radii of the galactic center. In addition to their role as weapons of last resort, the Halo installations are also research facilities, mainly for the study of Flood.
The Halo Array is a network of seven ring-shaped artificial worlds created by the Forerunners in order to kill all sentient life within range of the array, virtually the entire galaxy. Rather than as a weapon of war, these were used as a last resort against the Flood, a parasitic extra-galactic species that threatened to infest every lifeform in the galaxy. By activating the rings, the Forerunners denied the Flood access to these lifeforms, effectively starving the species to virtual extinction, though it clung to life in a number of Forerunner facilities, including some Halo installations. The two known Halo installations, referred to as Alpha Halo and Delta Halo respectively, orbit gas giants, though it is not known whether this is the case of all the Halos, and Alpha Halo maintained an atmospheric mining facility until it was destroyed by the Covenant. Though separated by thousands of light-years, the seven installations are networked together and capable of remote activation at the Ark, an installation outside the galaxy. Each Halo installation has a maximum effective range of 25,000 light-years. The exact means the array uses to conduct this "mass sterilization protocol" are unknown, though it is assumed to utilize energy in some type of form.
The Halo Array in its entirety has been fired once in known history, approximately 100,000 years ago, by the Didact in order to stop the Flood from overwhelming the galaxy. Despite being the network's builders, the Forerunners were killed in this last suicidal attempt to save the rest of the universe from a worse fate. The array has nearly been activated three times since; Alpha Halo was almost activated by the Master Chief before being deactivated by Cortana, while Delta Halo was activated by Commander Miranda Keyes, forced by the Jiralhanae Chieftain Tartarus, and Sergeant Johnson was forced by the Prophet of Truth to activate the Ark. None of these attempts were successful. A replacement Halo for the destroyed Alpha Halo was activated outside of the array's range, eliminating the freed Flood, but did not activate the rest of the array.
Each Halo Installation is given a designation number, from 01 to 07, and is overseen by a Monitor. The Monitors are given control of the Installations' Sentinels, Constructors and Enforcers, and are responsible for containing the Flood test subjects in their research facilities and protecting the Halo from intruders.
The Array also encompasses an installation known as The Ark or Installation 00, from which the array can be remotely and safely activated while out of firing range, and able to survive the effects. The Forerunners also built active Shield Worlds, Micro Dyson Spheres encased in a slipspace field with their own star in the center and a habitable environment, as a means to escape the effect without having to leave the galaxy.
The Halos were constructed by the Forerunners a few hundred years after the Flood were discovered by the Array's makers, because their builders saw that their current isolation and containment procedures were not adequate to stop the spread of the Flood. Little is known of their extensive history, (At least one was functional for 101,217 local years), but after exhausting every other tactical option, their creators had no choice but to activate the Halos' main weapon, and terminate all sentient life in the galaxy.
The seven Halos then remained relatively dormant for one hundred thousand years (though at least one of them experienced a major Flood outbreak; and reference given to brief visits by other species by 343 Guilty Spark in "Conversations from the Universe") until one of them, Installation 04 (known amongst fans as Alpha Halo), was discovered by the theocratic alien superpower, the Covenant. The Covenant, who revered the Forerunner as gods, believed that the Halo's main weapon was actually a source of "Divine Wind" meant to propel them on a path they called the "Great Journey". During their investigations on Installation 04, the Covenant encountered the Flood, however they quickly, but only partially contained the outbreak. After the arrival of the Human vessel UNSC Pillar of Autumn, Captain Jacob Keyes accidentally broke the containment, thinking to find a large Covenant weapons cache. A massive four way melee ensued between the Covenant, Sentinels led by 343 Guilty Spark, the Flood, and the Humans. This finally culminated when the Human Super-soldier known as Spartan 117 (Master Chief) detonated the fusion engines of the Pillar of Autumn on the surface of the Installation, which compromised the structural integrity of the ring, and, due to the gravitational momentum and inertia still perpetuating the ring in orbit around Threshold, tore the ring apart. The remains of installation 04 were scattered as debris in space, both large and small. The explosion sent massive chunks of the ring careening into the moon Basis, as seen in the Halo 2 Multi-Player Level "Burial Mounds," due to the constant bombardment of small objects from the ring as seen in the sky on that map. Other large sections of the ring are seen in the opening scene of Halo 2, drifting in space around Threshold. It is also highly likely that large portions of Alpha Halo debris were drawn into Threshold, never to be seen again. 
Later, a second Halo, Installation 05 (known amongst fans as Delta Halo), was discovered. On this Halo, the Flood had already been released and much of the Installation had been compromised, including the Ring's monitor, 2401 Pentinent Tangent. And owing to this, a similar conflict erupted. This conflict ended in the Flood manifesting a Gravemind and successfully escaping the ring aboard infected Covenant ships. The ring was nearly activated by Tartarus, a Brute Chieftain, but Commander Miranda Keyes pulled the Index out of the Installation at the last moment, causing the six remaining Installations to go into "standby mode", ready for remote activation from the Ark.
The Humans returned to Earth and resumed their fight against the Covenant, who had uncovered a large structure on Earth, which they assumed to be the Ark. The Covenant set the Forerunner Dreadnought down in the center of the structure, activating it. To everyone's surprise, the structure was not the Ark, but a generator of a portal to the Ark, a massive artificial world located outside the galaxy. Human and Elite forces then went through the portal to the Ark to try and stop the Covenant from activating the remaining Halo rings remotely. It was then that the Flood Gravemind appeared, having turned the Covenant planetoid city High Charity into a Flood hive. The Hive crashed onto the Ark, releasing the Flood.
At this point, it was revealed that the Ark had been constructing a new Installation 04, to replace the one which was destroyed by the Master Chief. This new Installation was only days from being complete. The Humans eventually decided to activate the new ring to stop the Flood, which in turn destroyed itself and damaged the Ark. It is unclear how the system was affected by this, but if the Ark was rendered inoperable, the Halo system will have been irreparably damaged by the destruction of Installation 04 and its Monitor (343 Guilty Spark).
The Installations are designed to be run by advanced Artificial Intelligence constructs specially purposed by the Forerunners. The highest Intelligence on each Installation is a single Monitor. The Monitor's task is to ensure that the Installation's Constructors, Sentinels and Enforcers repair, maintain and defend the ring from damage, contain the Flood specimens, and ensure that their own Installation is ready to fire on demand, including running activation simulations.
In addition, each Installation contains less intelligent constructs, known as Sentinels. The Sentinels serve virtually any purpose necessary to ensure that the Halo functions properly; they are also capable of combating small Flood outbreaks. Should a major Flood outbreak occur, heavier automatons, such as the Enforcers and Sentinel Majors will be created to assist the Sentinels. In the meantime, Constructors are also created to ensure that the Forerunner structures on the Installation are kept in optimal shape, and that they are not damaged by conflict or weather.
Save for the Monitor, all automatons can be constructed at specialized Sentinel Production Facilities that float high in the Halo's atmosphere. These Facilities have never been seen (except briefly in the sky above the level Sacred Icon, one can be seen crashing as you exit near the end of the level, you then fight through the remains of it in Quarantine Zone), but they seem to contain limitless materials for constructing any number of automatons.
The Halo installations were also designed to be habitable, and support a wide range of environments, habitats, ecosystems and climates. The two known installations, Alpha and Delta Halo, both supported warm, temperate forests that were both deciduous and coniferous, swamps, and cold, snow covered tundra environments. The Ark had climate ranging all the way from tundra, to forest, to desert. It is unknown whether this range is shared by all Halo installations, but the similarity of these environments and that of Installation 00 makes it likely. However, if one observes the holograms of the installations after stopping the Ark, one will see that the rings have different surface features and may have different atmospheres as well.
While the terrain may appear to be naturally formed at first glance, it is actually artificially constructed. Strato-Sentinels extract raw materials from the source, processes them in transit, and deposits building materials at the installation. Four huge terraforming factories then move across the face of the installation to "skin" it with landmasses and bodies of water. These factories also holds in the ring's nitrogen-oxygen atmosphere, which is then leaked out to the surface, eventually pouring through the super-structure, and tugged in place by centripetal force. The installation's buildings are then build by Constructors and Sentinels. An intricate layer of metallic panels is laid down several meters below the surface of the terrain, upon which rock, soil, and eventually vegetation is added. Slopes, hills and mountains can be created by sculpting these panels. Some features, such as landslides, are the result of time, and have been formed naturally. Others, such as the myriad network of tunnels and cave systems that riddle the Installations internal structure, are travel conduits used by the rings Sentinels and Enforcers. There are also some life-forms that live naturally on the Halos. Whether these were created by the Forerunners or were brought to the Halos by them is unknown.
Each Halo has several assets that are conditional to all Installations. Each contains a Control Room located somewhere upon its inner surface, from which a Reclaimer must manually insert the Installation's Index to activate it. Each also possesses a Library, a large structure protected by an energy shield, where the Index is housed and protected by Sentinels. The index itself is a semi-solid holographic representation of the data needed to activate the ring, and can only be inserted successfully by a Reclaimer.
Installations also possess Cartographer facilities, holographic representations of the Installations inner passages and networks, to be used as a navigational reference for exploring the Installation. Flood Containment Facilities are also standard, used by the Installations Monitor to conduct research on and observe the surviving Flood specimens, also protected by Sentinels to prevent outbreak.
Installations also contain advanced teleportation grids, allowing instantaneous transportation for a Monitor or Reclaimer to any place on the Installation. These grids can not be controlled by the Flood but the Gravemind is able to use it by controlling 2401 Penitent Tangent. For regular Sentinels, the Installation is riddled with tunnel and cave networks for access and transportation. The Sentinel portals are connected to factories via tunnels.
The Halo rings get their "gravity" through centripetal force. As the ring spins, centripetal force pushes all spinning bodies away from the center of rotation. The ring is uniform, and because of this all points are the same distance from the center, therefore centripetal force applies evenly. This force also pushes objects on the surface of the ring away from the center of rotation, or pushes them down on the surface of the ring, translating into gravity. When the UNSC Pillar of Autumn detonated, it severed the ring. No longer a full loop, the centripetal force applies in much greater strength on the newly-formed weak point, causing the ring to rip itself apart.
As revealed in the new Halo encyclopedia, when activated, the Halo rings would wipe out all sentient life within three radii of the Milky Way Galaxy's center by sending out a harmonic frequency, targeting certain cells in the nervous system, which includes, but is not limited to, neurons.
The Halo Array is complex, but each can be activated individually by a Reclaimer, also activating the other Installations, or they can all be activated simultaneously from The Ark. In the event of a major Flood outbreak, the Installation's Monitor will seek out a Reclaimer if available, who they will enlist to aid them. It will teleport them to the Installation's Library, and assist the individual in retrieving the Index, needed to activate the Installation. The Monitor then stores the Index within its data arrays for safe transportation, lest the Reclaimer fall prey to the Flood before they can activate the Installation. Once at the Control Room, the Reclaimer must insert the Index into the control panel in order to begin the activation sequence.
The main weapon is amplified by Phase Pulse Generators, three of which are known to be close to the Control Room. It is unknown exactly what means the Halo Array employs to kill sentient lifeforms over non-sentient life. Each Installation has a maximum effective radius of 25,000 light-years, and is apparently of the "highest possible threat assessment from biological contamination and radiological annihilation". The pulse is designed to kill all sentient life in the Installations three-dimensional radius, with the entire array covering the entire galaxy. The only known way to avoid the effects of these pulses is to escape to a Shield World, located in slip-space and therefore immune from its effects, or to escape to Installation 00, which is out of range. Once activated, the other installations will fire simultaneously, eradicating all sentient life in the galaxy, thus starving and killing the Flood.
The Halos apparently fire in a large burst of energy that continually increases in diameter until it reaches its maximum range, as seen in one of the final cutscenes of Halo 3.
The "Ringworld" of Halo is much smaller than Larry Niven's Ringworld. While the diameter of Niven's ring world is close to the diameter of Earth's orbital of 300,000,000 km (Able to rival the sun in surface area), the 10,000 km diameter of Halo is much closer to the diameter of Earth itself, which is 12,756 km. Ringworld has a star similar to our sun in its center, and Halo is orbiting a planet, but does not encircle it. Alpha Halo is positioned in the 1st Lagrangian point between Threshold and its moon Basis.
Besides the very beautifully sculpted landscape on Halo's surface, the sky is also worthy of attention. Depending on the viewpoint, you can see a star, a moon (called Basis), a gas giant (called Threshold), and the band of Halo itself, which looks like a street into the sky.
In the course of Halo: Combat Evolved, neither Threshold nor Basis is reachable, all action takes place on, in, or near the Halo installation. In Halo 2 there is a multiplayer level on Basis and a campaign level on a gas mining facility in Threshold's atmosphere.