Do you play through the same campaign in co-op as you would in singleplayer?
And does it make it easier? Or is co-op some mindless killing like Resistance 2 co-op? Because I'd really want it to be the former, it reminds me of Diablo II's atmosphere.
Basically, once you beat the first level in the game, you can summon people into your world by activating a blueeye stone you find on the ground. Once you find your own blueeye stone, you can lay it down and be summoned into other people's worlds to help them. Co-op is the same as singleplayer, and is one way to reduce the difficulty if you're having a hard time beating a level or boss. It's a little more complicated than that once you start considering soul/alive forms, but that's a good basic explanation.
Demon's Souls Official Thread | Currently playing: Left 4 Dead 2, LittleBigPlanet 2, Magicka