Spartans-

Taken from their parents at young ages, Spartans are chemically enhanced supersoldiers, part of an elite military program to help stop the Covenant onslaught. Many Spartans died during training, and the few left have been fighting a loosing battle. Equipped with MJOLNIR  battle armor and trained to deal with situations that would be deadly for any marine, Spartans were the best of the best. During the Attack on Reach, all remaining Spartans except John, Linda, and James were defending the planet when it was glassed by the Covenant. While the majority of the planet had been burned to a cinder, the UNSC headquarters had been spared the destruction, and the remaining Spartans regrouped there. At the time of Halo 2, four Spartans are combat-ready (including the Master Chief), and one is MIA.

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Humans-

The Human Race has fallen to it's knees against the wrath of the covenant . As humanity grew, it ran low on resources, and thusforth had to colonize other planets. As the colonization ended, humanity did seem to be the only race in the galaxy. However, an unknown race began attacking human colonies, and gained control one by one. The Covenant, as the came to be called, continued their onslaught against the humans until they destroyed Reach. After Reach, humanity built defense platforms across earth's space-regions, and fought the Covenant during the coming attacks.

Helljumpers-

Before the Spartan project, Helljumpers were known as the best of the best. Helljumpers are best described at the Rangers of Halo. They wear full body armor that resembles the Mjolnir battle armor, and are frequently in the front lines of combat or behind enemy lines.

Covenant-

The Covenant are multiple races that reside under one faith. Begun by the race known now as "The Prophets", a war was waged between the Prophets and Elites. When the Prophets won, the Elites were "Absorbed" into the Covenant. Then the Grunts, Jackals, Hunters, Brutes, Engineers, and Drones were defeated and absorbed. It is only assumable, however, that if the Covenant were to defeat the humans that it would mean total extinction. Here's the Non-Combatant guide to Covenant Forces.

Grunts-The Covenant's "cannon fodder", Grunts are generally weak and pose no serious threat. However, when led by an Elite, they will stand and fight. They use a breathing apparatus that supplies them with methane, their only breathable substance, and can be frequently found sleeping when they should be on guard. Thousands are sacrificed during combat, but in groups and led by Elites, Grunts can be a dangerous opponent. Yellow-armored Grunts are recruits, and are usually the most easily killed. Red-armored units are veterans, and Spec-Ops Grunts are the best, and the most dangerous, of the variations. 

Jackals-Birdlike and annoying, Jackals are superior to Grunts, but still very low in the Covenant's respect scale. Usually found wielding an energy shield and a plasma pistol, the Jackals can be a formidable oppenent if trained well. Jackals are also the Covenant's snipers.

Elites-The backbone of the Covenant, the Elites are stong, agile, and deadly. Elites are almost always using a shield device, which collapses after several rounds, and are usually found leading a platoon of Grunts and Jackals into combat. Elites carry just about every Covenant weapon avalible, and high-ranking Elites wield Energy Swords. The Elite can also be found operating Covenant Banshees, Wraiths, and Ghosts. Armor color on Elites signals the rank. Blue is a regular, red is a veteran, and white is a commander. Elites are promoted based on casualities they inflict.  Select Elites become the Prophet's Guards.

Brutes-Brutes are, if anything, the enforcers in the Covenant. For any heritic who is captured, severe tourture awaits at the hands of these animal-like Covenant. The Brutes usually carry out the "dirty work" of the Covenant Prophets, and can be found weilding Brute Plasma Rifles and Covenant Carbines.

Hunters-If you can look up "heart-pounding" in the dictionary, you could well see the face of one of these. The Hunters are huge, deathly creatures, with a plasma cannon welded onto an arm. They also have an extreme amount of strength, they can pick up a Marine and toss him around like a rag-doll. Covenant deploy them like machines, not units, and always work in pairs. The only weak point on a Hunter is the orange-ish flesh on it's stomach.

Prophets-Prophets are religious leaders among the Covenant, not fighting units. They're word is what the Covenant follow. To the death. Prophets like to float on hoverchairs, and are nearly constantly guarded by Elites or Brutes. The three most infulential of the prophets are Truth, Mercy, and Regret.

Flood-

The Flood are best described as an infestation. Once released, they overtake whatever possible. There are three "forms" of the Flood. A parasite, a humanoid form, and a carrier form are the three different types of Flood identified today. The Flood, in order to survive, must rely on a host. In short, they must take over another's body. The parasite form takes this stage. The captured then becomes deformed to nearly the point where you couldn't tell human or something else. Parasite forms that don't enter a host can do one of two things. Enter a slain Flood humaniod form to revive him, or become part of a carrier. The carrier holds several new parasites inside, and explodes when it comes in vicinity of a target. A parasite cannot enter a slain humanoid form if the slain has been shredded by grenades or an Energy Sword.