Halo consists of an intense, story-driven single-player campaign and a multiplayer mode. The campaign is a good 12 hours long at the normal difficulty setting, and the dynamic nature of the battles, along with the multiple, well-balanced difficulty settings, gives it good replay value. The multiplayer component only supports up to 16 players and includes a bare-bones integrated server finder.
Halo is famous for integrating powerful, fun-to-drive vehicles with the on-foot action, and this is what distinguishes its multiplayer component from that of other shooters, though some other PC shooters have also integrated vehicles more or less successfully since the original release of Halo.
The most impressive aspects of Halo are its friendly and enemy artificial intelligence, its physics for vehicles and explosions, and its weapon design. These elements are what contribute to the outstanding quality of the action, and though they are technically the elements of a 2-year-old game, they remain unsurpassed by just about any other shooter. Halo isn't a squad-based shooter, but in those instances when you're fighting alongside fellow marines, these troopers will naturally look to you for guidance and provide you with covering fire.
Surprisingly, Halo looks really great on the PC--if you can get it to run smoothly. The creature and vehicle design is top notch--there are clear thematic design differences between the steely, utilitarian human designs and the sleek, purple-hued, almost oily-looking alien constructions. The weapon effects, explosions, and vehicles are especially impressive, and the enemy animations are very well done, too.
Halo is still a tremendously entertaining game on its own merits, and it can easily be placed in the same category as other all-time greatest shooters, like Doom and Half-Life.