Halo 3: ODST is a new game in the "Halo" saga that lets people experience events leading up to the epic story told in "Halo 3" through the eyes of an ODST (Orbital Drop Shock Trooper), as they search for clues leading to the whereabouts of their scattered squad and the motivations behind the Covenant's invasion of New Mombasa. The release adds a new dimension to an all-encompassing universe that gamers around the world have known and loved for close to eight years.

A widget on Bungie's website tracks how many people have played Halo 3 ODST, so on the first day it tracked roughly 950k players, people who obviously just bought the game.

In addition to a brand new campaign, “Halo 3: ODST” introduces a new cooperative multiplayer mode called Firefight which allows “Halo” fans to experience co-operative gaming in a whole new way: they will be fighting as members of an ODST squad against waves of increasingly challenging enemies across a variety of battlefields. This experience is made even more intense with the integration of “Halo 3’s” “skulls,” which progressively activate as time goes on to alter the gameplay and up the challenge. Players will battle through wave after wave in search of high scores, Achievements and bragging rights.

“Halo 3: ODST” also includes a second standalone disc with three new “Halo 3” multiplayer maps, all original “Halo 3” maps, as well as the “Legendary,” “Heroic,” and “Mythic” Map packs for a total of 24 maps.

Halo 3: ODST will deliver a completely new campaign, a new cooperative multiplayer experience called Firefight, the complete “Halo 3” multiplayer experience on a standalone disc, and new competitive multiplayer maps for “Halo 3” you won’t find anywhere else. Microsoft and Bungie today revealed the remaining three multiplayer maps which are sure to get fans around the world excited.

Key Features:

• All-new campaign content. The "Halo 3: ODST" campaign explores mysterious events leading up to the Master Chief's return to New Mombasa in "Halo 3." Experience the fate of Earth from the brand-new perspective of a special forces ODST (Orbital Drop Shock Trooper), adding a unique outlook and new gameplay elements to the events that unfold in the trilogy's finale. This new chapter captures the same great feel of previous "Halo" experiences while offering fans new tactics, tools and weapons to defeat the Covenant.
• A new hero: Prepare to drop. ODSTs or "Orbital Drop Shock Troopers" are highly-trained special forces operators. Known for their courage, valor and willingness to tackle missions no sane soldier would, ODSTs pride themselves on being the first unit deployed into hostile situations, often in one-man drop-pods that burn through planetary atmospheres at break-neck speed.
• Expanded multiplayer experience. No "Halo" game would be complete without groundbreaking multiplayer content. "Halo 3: ODST" introduces an entirely new way to play "Halo" with Firefight, a cooperative multiplayer mode included on the campaign disc that lets up to four people jump immediately into combat. As an ODST squad, you will face increasingly challenging waves of enemies together, striving to earn the highest scores possible, while various "Halo 3" "Skulls" are progressively activated. The experience is intense and ever-evolving. "Halo 3: ODST" also includes a separate, standalone multiplayer disc which includes three new "Halo 3" multiplayer maps, all original "Halo 3" maps, as well as the "Legendary," "Heroic," and "Mythic" Map packs for a total of 24 maps.
• Ever-growing community options. Players will continue to enjoy all the great "Halo 3" technological innovations, including Saved Films, screenshots, Campaign Scoring and four-player co-op over Xbox LIVE. All of the new "Halo 3" multiplayer maps fully support the Forge editor, offering near endless gameplay customization possibilities for fans.
• Key to "Halo: Reach." As a special bonus for fans, all copies of "Halo 3: ODST" will provide access to the "Halo: Reach" multiplayer beta when the beta goes live. More information about the "Halo: Reach" multiplayer beta will be shared at a later date.