What is Halo Reach and why does it matter?

[coolestguyever]

My understanding is that ODST was only an expansion for Halo 3 (which costed the full $60) and not a full fledged "Halo 4".

And Halo Wars was a completely separate RTS game.

The reason people think reach will do so well is its basically Halo 4 and not an expansion. I don't think it will do Halo 3 numbers but it will certainly do well.