Okay, so I'm gonna try to make a concrete definition, not in favor of haters or fanboys. So, Halo is a video game, can we all agree on that? Good, here we go...
Halo:Combat Evolved (CE)
So, the main character,
Master Chief, is a human/cyborg guy who's really tall. So, he's trying to save earth from a bunch of aliens, called The Covenant(weird name right?). Anyway, while he's slaughtering aliens, The Covenant released some kind of parasite call The Flood which basically turns everything into zombie type things. Master Chief blows the spaceship that the Flood are on. Oh yeah, most of the game takes place on a circle thing
floating in space called the Halo(hence
the name of the game). So, Master Chief saved the galaxy, right? Right?
Halo2
Okay, awww crap. I haven't played this game in a while. There's not much you need to know about this game besides Master Chief said 'FINISH THE FIGHT'(The thing they say every 5 minutes in Halo3)
Halo3
Chiefy's back and he's ready to kill more aliens. In Halo3, Chief (SPOILER ALERT!!) blows up Halo and escapes in a dramatic car getaway level. Chiefy also drives the car into a spaceship to fly away with his Starsky and Hutch-esque buddy Arby. Oh yeah, there was also a bitchy computer lady named Cortana who whines all the time throughout all 3 games.
What a bitch.
Halo3: ODST
In this game you play as an ODST
Halo Reach
I only played the beta but I'll update this once it comes out.