An amazing game that has aged even better.
Halo is probably the meeting-point between the older shooters of two decades ago, and today's current fare. You can only carry two weapons and a total of 8 grenades (four of each type), and the regenerating shield mechanic is present. However, you have an actual health bar that doesn't inexplicably regenerate over time, requiring you to find medkits. The graphics are colorful and enjoyable to look at, rather than today's (and regrettably, the later Halo titles') photo-quality renditions of "industrial grey" and "muddy brown".
Gameplay is enjoyable; it presents a stiff challenge without ever being frustrating on Normal difficulty, and Heretic and Legendary difficulties are always available to those looking for more. Your computer-controlled allies, the Marines, are noticeably useful most of the time- a rarity in games that feature AI companions.
There's a large variety of vehicles and weapons available, as well as aliens to kill with them. The short, stocky Grunts can easily be mowed down by the handful with some skillful pistol work; but difficulty soon ramps up as you're required to face bird-like Jackals with energy shields, Elites that can turn invisible and wield instant-death plasma swords, and heavily armored Hunters with the fearsome Fuel Rod Gun.
Vehicle sections are incredible as well. Driving a Warthog all-terrain vehicle with a mounted machine-gun is a blast, and dogfights in the enigmatic alien Banshee are tense showdowns. There's more vehicles available to drive too, and a variant of the Warthog vehicle with a mounted rocket launcher is only available in multiplayer.
Overall, Halo is a fun game, and an amazing example of the FPS genre as a whole. Challenging while not frustrating, innovative without being unintuitive, and an incredible multiplayer experience; all of these describe Halo.