I guess i didn't explain it too well.
Diablo fans know how the single player and multiplayer games are basically identical, but multiplayer has more people. Like... when you play Diablo by yourself, you progress through the acts, completing certain things. It's the exact same thing online, but now you got 5 people completing the same quest.
NWN works in this way too. The official campaign can be played online and offline, and it's the same thing, just with more people playing online. Of course, a big part of NWN is how you can build your own worlds, and since the game just came out, we haven't seen many. But in the coming months, we'll probably see some great modules, and play a lot on those.
And yes, online play is free. Just need an account at
http://nwn.bioware.com which you can get now, or register when you first try to play online.