NWN is based totally off of 3rd Ed. Dungeons and Dragons. In this, players can choose whether to be good, evil, or neutral alignment and chaotic, lawful, and neutral stances within those alignments. It's much like in Baldur's Gate 2. Now, in multiplayer you will be able to be any combination of these. You will be treated differently in different towns, by different people, by different monsters perhaps... based on your alignment.
In single player, I'm not sure, since I'm not focusing on single player for NWN... but, I think you can choose any alignment there too. And you can play through the campaign based on your alignment, responding differently based on your personal views on life. Just like in Baldur's Gate 2 where you can choose whether to spare that double-crosser, or choose to smite him.
Try taking a look at
http://home.earthlink.net/~teluial/OGF/
It has many answers, as well as the official Bioware forum and FAQ for NWN, found here
http://forums.bioware.com/