Halo: Combat Evolved is a great game. Halo: Combat Evolved Anniversary is a better game.
Anniversary is a remastering of the original Halo (though you can switch back to the old graphics anytime you want), so everything is basically the same. Some people thought the gameplay should have been improved, but I was happy it remained the same. Maybe it’s not perfect, but it was good enough when the original game came out and trying to “fix” something that isn’t broken often causes more damage than anything else.
If you’re unfamiliar with Halo, it’s a science fiction first-person shooter where you play as a Spartan supersoldier called the Master Chief, in a conflict against the alien Covenant. Your goal is to explore the mysterious planet-like ring you’ve arrived at and find out what the Covenant are looking for.
It’s a great story with some surprising twists (though this long since the series began, they probably aren’t surprises to most people anymore), particularly one partway through the game that changes everything up.
Most people, however, play Halo for its multiplayer. Anniversary has online multiplayer as well as split-screen, with both competitive and cooperative modes, providing many opportunities to enjoy Halo multiplayer across a variety of maps.
Notice I said most people, but not everyone. Some people are fascinated by Halo’s story and lore. I’m one of those players. I play Halo primarily for its story. I read the Halo novels and love what they add to the universe. Yet when it comes to the novelization of the first game, I always recommend people play Halo: Combat Evolved Anniversary instead of reading it if they can.