Artificial gravity

Artificial gravity is a technology employed by the UNSC, Covenant and the Forerunner, to generate gravity of varying strengths in an otherwise zero or low-gravity environment.

Centrifugal force
A number of UNSC ships employ rotating sections of the hull to provide centrifugal force on the interior of the hull to allow comfortable movement in "gravity." Carriers are able to adjust the rate of rotation, allowing more or less gravity, and a number of other ship classes, such as Halcyon-class Cruisers, also used rotating sections before the introduction of more advanced gravity plating. The rate of spin for many of the smaller UNSC ships would have to be quite low (2 RPMs or lower) to prevent the crew from getting dizzy and nauseated due to the Coriolis effect. This means that the "gravity" produced by the rotating sections of most UNSC ships would be well below 1g. Space stations and colonies at least a mile in diameter would likely be able to produce 1g. But this style of artificial gravity fails to explain how an elevator can run through it and how you traverse out of the rotating section, something that occurs multiple times in the novels.

The Halo Installations rotate, and may use centrifugal force to provide at least part of their Earth-like gravity, possibly employing the same gravity generation techniques used by the Covenant, or another method, to generate the rest.

Linear acceleration
A ship accelerating at 1G would be able to simulate gravity by using inertia. If certain decks within a ship were oriented to where "up" was the bow and "down" was the stern, then constant acceleration (or deceleration) at 1G would simulate gravity. Many UNSC ships probably used a combination of centrifugal force and linear acceleration to provide simulated gravity prior to the introduction of more advanced technology. It's likely that space stations and some ships would continue to use these technologies, as they would not require an energy source beyond the ship's own acceleration or rotation.

Covenant
Almost all Covenant vessels employ some sort of artificial gravity mechanism that does not involve rotating hull sections or acceleration. Instead, their method allows for the generation of actual gravity. It has been speculated that they may use diamagnetism, but this is very unlikely: though there would be no known adverse health effects, it would mean that the entire ship would have to be made of non-magnetic materials, have heavily shielded electronic equipment and the crew would not be able to use anything made of metal. The use of ultra-dense materials which generate their own gravity has been suggested, but this would make the ship incredibly heavy and massive. Considering the UNSC still use some form of reaction/rocket propulsion, it seems rather unlikely that this would be the case.

The most likely explanation is that the Covenant have developed some form of directional gravity emitter that generates a beam of gravitons. This would mean the Covenant are able to generate gravity without mass. It is possible that the Covenant have figured out a way to convert energy into graviton particles and direct them in much the same way as a beam of light; this seems to be the basis for the Covenant's gravity lift. The production of "antigravitons" would create a repulsive effect, which may be the basis for some of their antigravity drives. All this suggests the Covenant are able to produce gravity and antigravity as easily as humans can produce positive and negative electric current.

Human
While some human ships used rotating centrifuges to mimic gravity, humans did possess a form of artificial gravity even prior to their contact with the Covenant. This is demonstrated by the DCS freighters enabling artificial gravity through "anti-gravity units" and the orbital platform Tiara having artificial gravity without the use of rotating sections. Additionally, many ship classes commissioned before the Human-Covenant War, including Halcyon-class Cruisers and Phoenix-class Colony Ships were known to have a form of artificial gravity other than spinning sections.

It may be that Antigravity plates (as seen in Halo: Ghosts of Onyx, said to have not "panned out") were related to tests with artificial gravity. Since the Antigrav plate had a crushing or repulsive effect, and it was described as generating a beam, it most likely generated directed antigravitons.

UNSC starships clearly have a form of anti-gravity technology, demonstrated by their ability to hover in planetary atmosphere without rocket propulsion being directed below. It's possible that only very large power plants can generate the power needed to produce antigravity, and the antigravity plate may be an early attempt to miniaturize the system found on starships. This would mean the human and Covenant technology were not that far apart in this regard, with humans lacking the technology needed to miniaturize gravity/antigravity generators.

Trivia

 * In all of the Halo games, all human vessels or stations seem to employ sophisticated, unseen artificial gravity generators to simulate gravity, as opposed to rotating sections. Originally, it was widely assumed that the UNSC gained their current artificial gravity technology from the Covenant, and had to use rotating centrifuges to mimic gravity prior to this. According to Halo: The Fall of Reach, human ships were still undergoing tests with reverse-engineered artificial gravity by 2552, while newer sources such as Halo: Contact Harvest portray artificial gravity present even on automated freighters.
 * Eugene Podkletnov has claimed that he has designed a device that can create artificial gravity. In contrast, a group funded by the ESA claim to have developed a device that reduces the effect of gravity. The validity of these claims are unclear.
 * Artificial gravity is a popular concept among science fiction novels, television shows and films, but attempts to explain their workings are usually implausible, involving gravity field generators.

Internal

 * Anti-gravity technology

External

 * Gravity on Wikipedia
 * Artificial gravity on Wikipedia