Wow, if you're LEET at this game, you're LEET!
The single player sets you off running from the covenant (aliens) and crashing on this ringworld called "Halo". You are the master chief, a technologically advanced super soldier in the Navy. Only here the navy is made of space ships. Eventually you cross over every type of terrain (except desert) and destroy Halo only escaping with an artificial intelligence entity, known as Cortona, and your life.
Multiplayer is really why Halo is so popular. The fact that in most games you start with a pretty powerful gun keeps the game perfectly balanced, only the skilled players come out on top. There are a diverse amount of gametypes going from "Oddball" where you hold a skull for points, to "Slayer", which is basically just deathmatch, all the way to "Race" where you talk the game's vehicles to flag points positioned on the map. Race is my favorite gametype, but has taken alot of flack in the past because people found it boring...
Overall, this is the greatest game ever, bar none!