>>81
Here's a rundown of what I know about the Halo universe:
* The Forerunners are a technologically advanced non-human civilization. They live life and create technology and do whatever.
* A parasite of some sort enters the Halo galaxy and start to spread. They are referred to as the flood because of the nature of their infection rate.
* The Forerunners build a collection of Halo structures for the purpose of studying the parasite and as containment for the spread of the parasite when it gets out of control. Activating the Halo structures would contain the growth of the parasite by depriving them of their food source - that is "killing all sentient life" within the galaxy.
* The forerunners decide that the parasite's spread was beyond their control and so implemented protocols to contain the spread. The protocol was basically to move somewhere else (the great journey) and activate the Halo structures (the purification). All sentient life within the Galaxy die by the Halos. The parasite is deprived of their food source and their method of reproduction.
* The descendants of the Forerunners live on. The Forerunners promise salvation from the parasite to their descendants through the containment protocols. The ancestors call themselves 'the Covenant' after the Forerunner's promises.
* The Covenant somehow causes the parasite to spread. The Covenant start the great journey but need to find the Halos.
* Humans develop feasible interstellar travel and colonize the galaxy. The humans start the SPARTAN project to help deal with any military conflict. Humans and the Covenant discover each other. The Covenant war with the humans (I don't know why). The Covenant kick human butt.
* Humans and the Covenant discover the first Halo (Installation 04). The Halo 1 story begins here.
That's basically the story of the Halo universe up to Halo 1. I've left out tons of trivial detail. The Halo wikia site is probably the best non-official Halo resource we have. You can find tons of Halo trivia there
http://halo.wikia.com/wiki/Main_Page .
All that detail makes up for a plausibly interesting story. If you view Halo shallowly though, Halo is yet another shooter with aliens and zombie-like parasites futuristic weapons. This is why I say the difference between Halo and other shooters is the story.