Plasma

In physics and chemistry, Plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma therefore has properties quite unlike those of solids, liquids or gases and is considered to be a distinct state of matter. Plasma typically takes the form of neutral gas-like clouds; for example, in the case of stars.

Covenant
The Covenant military uses plasma in a weapon form for a great effect, containing and guiding it in the form of a single bolt, beam, or continuous stream. Plasma-based weapons are the most common armament of Covenant infantry and vehicular forces, and are the main armaments of their warships. Smaller weapons are designed to deplete shielding or melt infantry armor, while heavier weapons can destroy vehicles or buildings. The largest are mounted to capital ships and are used for orbital bombardment and ship-to-ship combat, and can reduce a ship's hull to a pile of molten metal floating in space. When a plasma weapon is charging, high quantities of beta particle radiation are emitted.

UNSC
The UNSC uses plasma as an energy source and to cut heavy metals in factories, and attempted to produce it for use in their armed services, but apparently chose to abandon the project, instead falling back on projectile weaponry.

The Titanium armored battle-plate used on UNSC warships has a melting point of 1668 degrees Centigrade, however plasma, as said in the book Halo: The Ghost of Onyx is said to be around 3,000 degrees C. If the UNSC decided to use a tungsten alloy battle plating, then their ships' chances of survival could be increased substantially, since tungsten melts at 3,448 degrees C. However, it is possible that since the UNSC uses Titanium-A, an augmented state of titanium, that the melting point of their starship armor is much higher than suspected.

Forerunner
The only known Forerunner constructs to use plasma weaponry are the Sentinels.

Protection
Complete protection from plasma is generally difficult to obtain due to the plasma's high damage ratio. The armor used by the UNSC marines usually has a good protection from plasma, but only to an extent. As a result, a series of hits from a plasma weapon will literally melt or burn the armor, turning it to uselessness. That's why when a Marine is constantly hit in a certain place numerous times by plasma, the armor will give way and the person will have severe to life threatening injuries. The SPARTAN-II armor variant is similar the marines armor, being that it will eventually give way to the plasma. The added bonus Spartans have is the energy shield which protects from plasma although shields are susceptible to overcharged plasma.

Trivia

 * For gameplay reasons, plasma is less lethal to unshielded NPCs in the Halo games than in the novels. In the games, marines can take a barrage from a plasma weapon without being incapacitated, but in the books there are several incidences of plasma easily burning through both armor and flesh. This disparity is due to gameplay mechanics, rather than being a canon contradiction.
 * It should be noted that in Halo: Combat Evolved and Halo 2, the plasma on some of the plasma based weaponry, most notably the Energy Sword, can be seen even when Active Camouflage/Invisibility has been activated.