The story of Halo xbox game? Whats it about?

okay my nephews are always playing Halo the game for xbox and xbox 360. I play the game but have no idea what its about all i know is that the green guy is called "spartan" and you fight against aliens and predators. Can any one elaborate the different Halo and what is the story is about. maybe a short summary of each game Halo released like Halo 1, 2 etc?