"Halo Reach" takes place about a month before the events of "Halo Combat Evolved" where the planet Reach is under attack by the Covenant Empire.
I've played other Halo games before (Halo CE, Halo 2, and Halo 3), but this is the only Halo game I own. And it's awesome. The campaign is good. I like how it shows what happened before the first game and shows you how those events led to the defeat of the Covenant (as we see in Halo 3).
The graphics are great, the A.I. is actually really good, and the voice actors are pretty good. Also, multiplayer and Firefight are awesome, especially with being able to drive warthogs, ghosts, etc.
Also, no Halo game is complete without its amazing music. The theme music delivers that "war tone" feeling you would want in a game like this and the other games in the series. I also liked playing co-op campaign with either other people or just with a buddy.
Overall, "Halo Reach" is a great game, one of my favorites, and I give it an "A+". By the way, I also play "Call of Duty". I'm saying this, because I know there are a lot of people who play Halo and don't like COD, or vice versa.