Halo: Combat Evolved is a mostly average First Person Shooter that was scheduled for release on the PC, but Microsoft decided to buy the company and send their game (which had already, inexplicably, gained incredible amounts of hype) to their new console system, the Xbox. Halo was released on the Xbox in 2001. It was later released for the PC in 2003.
Halo is a simple game. Basically, the entire game revolves around "killing aliens" and "not killing humans", all the while navigating your way around a maze or terrain to get to a certain area, which the game passes off as "objective-based gameplay".
Your weapons in Halo are standard fare: you have your pistol, shotgun, assault rifle, and your weird alien guns. In addition, you have a rechargeable shield that protects you from a bit of damage. You can take cover while it recharges. Truly, this game lives up to its subtitle of "Combat Evolved".