I guess I'll copy the stuff I crappily translated out from Newtype + the DtB website over here.
I never realised there was a prelude in the official site though, that's where the animenfo synopsis was translated from. It seems that these "keiyakusha"="contractors/contractees" (I don't know which of the two it is) have got supernatural powers (this is mentioned in the scans as well) which is related to the Hell's Gate.
But what I want to know is, what's with all the Chinese? Surely they're not going to put something stupid like saying that the Hell's Gate is really a conspiracy of the Chinese government or something, right?
Anyway, the following info is taken from Newtype Jan. and also from the Darker than Black website both.
It's not all direct translations, I kind of threw them both together and picked out what I understood. I don't think I left out any details though.