The game is set in the near future, after Hell literally invades London. I know, you didn't expect that from the name at all. There are underground stations that hold refugees and those that fight back.
There are 3 groups of rebels, the Templars (holy avengers), Cabalists (using the demon's powers against them), and Hunters (ex-military ops). Each has two classes, the Templars with Guardians and Blademasters, the Cabalists with Evokers and Summoners, and lastly the Hunters with Engineers and Marksman.
The game plays as a mixture of FPS style gameplay, mixed with gaming style of Diablo 2 in the sense of acts, inventory and levels of difficulty. It borrows it's quest ideals from every MMO out there (WoW, CoH/CoV, FFXI, EQ etc).
Game Informer, a generally impartial reviewer, gave Hellgate London an 8.5/10.