What is HALO?
You're kidding, right? Halo is the brainchild of Bungie Studios. It's one of the biggest videogame series in the world, and arguably the best exclusive on the Xbox. That means you won't find a Halo game on any other home console - it's basically a reason to buy a 360. On top of that, Halo is also a huge multimedia phenomenon that's grown to include comics, figurines and novels - and maybe even a movie in the not-so-far-future.
The games themselves mostly take the form of First-Person shooters, pitting you as a rock-hard armoured space marine fighting against a fanatical alien race called The Covenant. We say mostly, because occasionally you get to play as a Covenant soldier, instead - while one of the Halo games, Halo Wars, isn't actually a shooter at all; it's a Real-Time Strategy game based on the back-story of Bungie's acclaimed sci-fi universe.
Before there was HALO...
...There was Marathon. No, not the old name for Snickers, but actually the series that Bungie made for the Macintosh prior to Halo.
Marathon was a critically-acclaimed first-person shooter which the Halo games openly reference with injokes, iconography and sci-fi subject matter - although Bungie insist that the two game universes are separate. First released in 1994, with sequels Marathon 2: Durandal and Marathon Infinity arriving in '95 and '96 respectively, the Marathon trilogy was one of the era's most advanced FPSs, but being a Mac-only release flew under the radar next to ID's all-conquering DOOM for PC.
If you want to try Marathon and witness Halo's spiritual heritage, you can download it here.