Informational Text from Microsoft's Site

Huge Weapon Variety
Vanquish your enemies with a vast array of Human or Covenant weaponry ranging from the stealthy (semi-automatic pistols, alien needler guns) to the fierce (rocket launchers and flame throwers).
Vehicle & Foot-Based Action
Tackle missions any way you choose; be it storming an enemy base, or taking the wheel or gunnery position of a powerful vehicle arsenal. Vehicles range from stolen Covenant flyers & tanks to Human buggies, dropships, and more.
Indoor & Outdoor Combat
Fight seamlessly in Halo's ultra-realistic indoor and outdoor environments as you hunt the Covenant in huge single player missions and multiplayer battles.
Intense Multiplayer Shoot-Outs
Form a team, pick a role, and fight cooperatively with your friends or battle it out together via intense split screen combat over a LAN.
Incredible Mission Variety
Fight the Covenant in dozens of missions as you uncover the dark secrets of Halo. Among your many objectives - attack enemy outposts, raid underground labs for secret technology, rescue fallen comrades, steal alien vehicles and weaponry, and snipe enemy forces.
Reality Bending Special Effects
Halo features the most advanced graphic system on the most advanced gaming platform in the world. Gunfire rips off the screen, explosions light up a living room, and environments bend the separation between your couch and fantasy. Rich Sci-Fi Experience
Halo transports gamers to a science fiction universe fresh out of a Hollywood movie with a detailed, twisting storyline, complex characters and cunning enemies.

BACKSTORY
For centuries, mankind has colonized the Orion Arm of the Milky Way galaxy. Though each world held dangers, the military-accompanying bands of intrepid colonists-carved out dominion over hundreds of planets. Human ingenuity, resourcefulness and bravery have literally tamed the heavens.
Now, in the 24th Century, a mysterious alien enemy-the Covenant-appears, striking without warning. Driven by a fanatical need to rid the universe of the "infection" of Humankind, Covenant forces mercilessly advance, crushing all in their path.
Soon, the once proud Colonial dominion of Earth is reduced to a handful of worlds.
For more in-depth information on the historical events leading up to this current predicament, simply click on any of the highlighted years below. Please be sure to check back often for the latest back story installment.

[NOTE: 'Early Conflicts' was the only entry in the 'highlighted years' section. One presumes they were/are planning to add other sections as time goes on.]

EARLY CONFLICTS
This period in human history is characterized by a series of wars between various governments and factions in our Solar System. Conflicts of particular historical importance include the Jovian Moon Campaign, The Rain Forest Wars, and a series of clashes on Mars.

[NOTE: The following sections were added on September 22, following their discovery on the Microsoft site.]

2164: INTERPLANETARY WAR
UN-sponsored military forces begin a pattern of massive buildups, culminating in the first real interplanetary war. After the successful Marine deployment on Mars, recruitment drives and propaganda tactics strongly bolster UNSC (UN Space Command) forces. UN forces defeat Koslovic (supporters of rabid Communist hard-liner Vladimir Koslov) and Frieden (a resurgence of fascism) forces on Earth, then begin a systematic and dedicated drive to crush their remnants on the various planets they hold throughout the system. At the conclusion, Frieden and Koslovic forces are defeated, in the face of a massive, unified and very powerful UN military.

2170: EXPANSION
The next 120 years is characterized by a period of restructuring as a unified Earth government is formed. Now, the victors must deal with a less-obvious, but equally serious threat: overpopulation and a massive military with no enemy to fight. In the postwar period, there are massive population surges and the overpopulation, coupled with the destruction and famine bred by the Rain Forest Wars, becomes threatening.

2291: FASTER THAN LIGHT
A secret team of researchers, physicists and mathematicians develop the Shaw-Fujikawa Translight Engine, a practical means of propelling spacecraft across vast interstellar distances.

2310: FIRST WAVE
The Earth government unveils the first in a line of colony ships; given conditions on Earth are deteriorating in the face of overpopulation, hitching a ride out to a colony becomes a highly attractive option. Additionally, the Earth government plans to attach military personnel to each colony, to help better utilize the massive (and expensive) standing fleets.
Because FTL travel is still fairly new and expensive, colonists and military personnel face a stringent regimen of physical and mental testing. In theory, only the best-qualified citizens and soldiers are allowed to colonize "nearby" worlds. This is the birth of the Inner Colonies; typically Inner Colonials are-later in the timeline-considered the elite, the best and brightest.

2362: THE ODYSSEY
The Odyssey is launched January 1, 2362. The lead ship in the colony vessels, the Odyssey-laden with troops and terra forming gear-spearheads the colonization of a new world. This sparks the first wave of human colonization.

2390: INNER COLONIES
Colonization of the Inner Colonies is fully underway with 210 human-occupied worlds in various stages of terra forming.

2490: THE BIRTH OF THE OUTER COLONIES
Expansion continues at a rapid pace, and by 2490 human space encompasses 800+ worlds throughout the Orion Arm of the Milky Way (ranging from fairly well-tamed planetary strongholds to tiny hinterland settlements). Outward expansion continues, and the Inner Colonies become a political and economic stronghold, though they rely heavily on raw materials supplied by the Outer Colonies.
During this period, the planet Reach (orbiting Epsilon Eridani, literally on Earth's front porch) becomes the primary Naval yard, a major producer of warships and colony vessels, as well as a training ground for covert operatives and Special Forces.

2530: THE COVENANT WAR BEGINS
On April 3, 2530, contact with the Outer Colony, Harvest, is lost. After failing to re-establish contact with Harvest, the Colonial Military Administration sends a scout ship, the Argo, to investigate. Contact with Argo is subsequently lost after arriving in the Harvest system.
The CMA dispatches a battle group to Harvest, consisting of three warships. The lead ship in the battle group, the Heracles, returns to Reach, badly damaged and with heavy casualties. The ship's commander reports that an alien warship with powerful weaponry was present, and had decimated Harvest, exterminating the colonists (and presumably destroying the Argo). The battle group engaged the alien warship, and was thoroughly routed. The Heracles-following the destruction of the other two ships in the group-jumped out of the system, but due to the damage she had sustained, took several weeks to make it back to Reach.
The Earth military goes on full alert, and begins making preparations to move in and retake Harvest. By December, a massive Earth war fleet under Vice Admiral Preston Cole mobilizes, one of the fastest such mobilizations of such a large fleet in the history of humankind.

2531: THE BATTLE OF HARVEST
Cole's war fleet engages the alien warship responsible for the decimation of the colony, scoring a victory (though the battle cost Cole two-thirds of his battle group). Only a last minute tactical inspiration turned the tide of battle.
After returning to Earth, Cole-promoted to Admiral-learns that a number of outlying colonies have been destroyed, leaving behind no survivors. Cole begins to move his fleet around, trying to intercept the invaders. Ground and ship-to-ship battles begin in earnest, raging throughout the Outer Colonies. During one ground engagement, Human forces capture one of the aliens. Before succumbing to his wounds, interrogators learn that the aliens refer to themselves as "the Covenant" (roughly translated).

2535: THE OUTER COLONY MASSACRED
In the course of four short years, Cole's forces are hammered, despite his excellent leadership and tactical brilliance. It is simply a matter of being outgunned; Covenant kill ratios tend to top four to one in ship-to-ship combat.
By 2535, virtually all of the Outer Colony worlds have been destroyed by the Covenant. The "Cole Protocol" is established by military order: all human vessels must ensure that Covenant forces do not find Earth. Human ships must-when forced to withdraw-not move on to an Earth bound vector, even if that means jumping without proper navigational calculations. If blind jumps are not possible, the ship's captain must order self-destruction if capture is imminent. In addition, it is also imperative that the powerful ship A.I. data cores not fall into Covenant hands, so part of this protocol involves either the removal or destruction of the ship's A.I. in extreme situations.

2536: THE SIEGE OF THE INNER COLONIES
Covenant forces begin to swarm into the Inner Colonies. For several years the war falls into a pattern: humans win isolated battles, typically during ground operations, but at a horrible cost. In space combat, the humans are losing at a precipitous rate, and one by one, the colonies fall.

2552 AND BEYOND: HALO
Covenant forces arrive at Reach and obliterate the colony. You (Master Chief) escape aboard the cruiser Pillar of Autumn and are the only survivor of your team.
In accordance with the Cole Protocol, the Pillar of Autumn's captain plots a random, long-distance jump, hoping to lure the Covenant fleet-dangerously near Earth-away from the human homeworld.
After deactivating the drive, the Pillar of Autumn drops into a distant, uncharted system. There is a Covenant fleet in the system as well, near a planet-sized ring-shaped construct, Halo. A brief-and lopsided-battle ensues and the Pillar of Autumn is severely damaged. An escape craft containing the surviving humans crashes on the mysterious alien construct.

Informational Text from Microsoft's Site (pulled September 14, 2001)

Heavily updated with information discovered by accident on September 21, 2001