Dark Souls, like Demon’s Souls, leaves you in a world without much to go on, except for the assistance (or hindrance) of player set messages left on the ground to guide you. Demon’s Souls had a great and unique multiplayer in which you can see the ghostly images of other players running about in their own worlds, to give you a sense of you are not the only one left, and Dark Souls follows that tradition.
Other Multiplayer features are invasions (other players attacking you), Kindling fires (which gives one more extra flask from others who kindle their own fires close in the network. You can also lay down summoning signs to assist others in their world as a phantom, or activate their own signs to summon up to 2 other phantoms to assist you in your fight for the current area.
A big difference from Demon’s Souls, is instead of the World/Player system, is the use of 9 covenants, which offer different gameplay styles. Some Covenants are PvP (Player vs Player) base, Some are Co-operative gameplay, and each Covenant is unique in their own way and different rewards. However, betraying your covenant can lead to consequences, or attacking/killing NPCs (non-player characters) can have dire consequences as well, so be careful!
Dark Souls, like Demon’s Souls, uses the souls you gather from enemies and such, as currency. Do you want to level up, purchase items, absolve your sins? It all requires souls, and you need to choose wisely what you will do with them.
The games unique approach to multiplayer is a very nice touch, and no other game series follows in their Multiplayer pursuit. Also, coming soon there is the Prepare to Die Edition, which will be available on the consoles and the PC version as well, so more content is on the way in the coming months.