Artificial gravity

Artificial gravity is a technology employed by the UNSC and Covenant, and Forerunner technology, to generate artificial gravity of varying strengths.

Centrifugal Motion
Prior to contact with the advanced technology of the Covenant, a number of UNSC ships employed rotating sections of the hull to provide enough centrifugal force on the interior of the hull to allow comfortable movement in "gravity." Carriers were able to adjust the rate of rotation, allowing more or less gravity, and a number of other ship classes, such as Halcyon-class Cruiser's, also used rotating sections before the introduction of reverse engineered gravity plating.

The Halo Installations apparently 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.

Covenant Artificial Gravity
In contrast, almost all Covenant vessels employ some sort of artificial gravity mechanism that does not involve rotating hull sections. Instead, their method allows for the generation of actual gravity, or alternatively diamagnetism, through artificial means, or by using ultra-dense materials which generate their own gravity, a means not beyond their level of sophistication.

Although the UNSC experimented with artificial gravity prior to First Contact, most attempts were unsuccessful, or produced mixed results. They generated an artificial gravity field, but were too dangerous and unpredictable to use in ships, with some passing into rebel possession. After contact with the Covenant, however, reverse-engineering Covenant technology allowed the UNSC to employ artificial gravity on most of its ships.

If the method used by the UNSC and the Covenant is indeed Diamagnetism, using the bodies own magnetic field to draw it towards the generator, then the health effects would be unclear, but likely detrimental. However, the use of dense materials with their own gravity would have relatively few side-effects, being actual gravity, but dramatically increase the mass of the ship, hindering propulsion.

Discrepancies Regarding Artificial Gravity
The first of such problems concerns whether the UNSC possessed non-centrifugal artificial gravity before contact with the Covenant. Certainly, the Pillar of Autumn was designed to be equipped with artificial gravity as seen in the games, but its class was first comissioned twenty-one years before the start of the Human-Covenant War, still incorporated a rotating section.

Halo: Contact Harvest also illustrates that human DCS freighters were equipped with some form of gravity. Although this may be based around rotation, description of the craft in the book appears to suggest that some form of generators are employed.

It is possible that in 2525 (when Halo: Contact Harvest takes place), the antigravity plates seen in Halo: Ghosts of Onyx were undergoing trial runs. It seems plausible that like General Howard Graves of the URF said, the plates were taken out of use shortly after 2525.

Trivia

 * 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.