Someone spoil Halo 2 for me

[wautd]

I picked up Halo 3 but since I've never played the second game,

Why am I falling out of the sky?
Who is this Arbiter character?
Anything else of importance I should know?

[Bounty]

http://en.wikipedia.org/wiki/Halo_2#Synopsis

And I'm sure the ending cinematic is on Youtube somewhere.

[Sarevok]

If you have not already try playing Halo 1. It gives you an idea of what Halo was like before fanboy wank and hype spoiled halo 2. Plus it will give you the full backstory on the Flood, the Halo rings and the forerunners.

[Grand Admiral Thrawn]

This site has cutscenes and level transcripts for Halo 1 and 2.

[SylasGaunt]

I picked up Halo 3 but since I've never played the second game,

Why am I falling out of the sky?

One of the Covenant prophets swiped an old forerunner ship to run the hell back to Earth and away from the flood. Master Chief managed to stow away and hopped out once they got back to Earth.

Who is this Arbiter character?

The commander of the Covenant fleet around the first Halo ring. Disgraced after the master chief blew it up, and given the job as the Arbiter in kind of the same context that you get Dwarf Slayers in warhammer, he's supposed to get himself killed doing something useful for the Covenant. Of course instead he ended up being part of the leadership of the Elites when they broke from the Covenant after they realized the prophets have been feeding them a line of horse shit regarding the halo rings.

The catalyst for the whole Elite split off though was when the Prophets replaced them with the Brutes after the Chief wasted one of the Prophet despite the protection of Elite guards and punched the shit out of him.

Anything else of importance I should know?

Depends on if you played Halo. If you played the first one you know most of the stuff about the Flood though now they've got a Gravemind since they ate enough people to develop one (central directing intelligence instead of space zombie mode). The Flood boarded the Covenant Capital of Higher Charity (which is a huge ass mobile space station) after some doofus woke them up on the Delta Halo discovered in Halo 2.

For the most part those sites given earlier should sort things out.

[DPDarkPrimus]

This site has cutscenes and level transcripts for Halo 1 and 2.

The Halo 2 cutscenes on the Legendary Edition extras disc of Halo 3 include various parts of in-level scenes as well, so it's more encompassing than just seeing the cinematic cutscenes in the archive, but the HBO archive is better than nothing.