WHAT!? You haven't bought it yet? tsk tsk. You are missing out.
Ok, here's what NWN entails. There is a single player campaign which includes 5 chapters(4 chapters + 1 Prologue). I have been playing for 2 weeks now and I am only on Chapter 3.
Unlike the previous Bioware releases (BG, BG2, IWD) You can only control your character plus (1) Henchman, if you decide to hire one. You only pay once for a particular henchman. If you stick with that henchman, and he dies, he goes into a sort of limbo at the Temple of Tyr. You have a magical stone that will teleport you back to the temple, so you can either heal up, sell / identify stuff you find, buy new stuff, pickup your henchman if he/she dies, or if you get in a tight spot, use your stone and you are teleported back to the temple. If you are the leader of the group, you will always have to pay a fee to go back through the portal from where you left. Keep in mind, if you used the stone to get away from baddies, then they will be there when you go back through it. If you happen to die in the game, you have the choice of either re-loading from a previous save file, or you can respawn back at the Temple of Tyr for a loss of 50 XP and 10% GP. If you die enough times, you can even drop a level. There are some areas in the game that if you die, you cannot respawn, so save often. ;)
You go through a series of quests (SP or MP) in the campaign, some of which have nothing to do with the plot of the campaign. The side quests are just for extra experience and adventuring, but fun none the less. One of the cool things about NWN is that you can play thorugh the SP campaign in Co-op MP mode. You have a plethora of choices for setting up a server(limit server only characters, local characters, passwords, etc..). You can play as the DM if you choose using the DM client(Seperate executable). Until new modules / missions are created either by yourself or another member of the NWN community, you are basically stuck with the NWN campaign as far as SP and MP are concerned. It may seem repetative, but most of the encounters(Monsters, and such) are randomized. The game randomly chooses the type and number monsters for you to fight based on the strength of your current party. Sort of a dynamic feature.
The toolset. The toolset is the bread and butter of NWN. You can design a module / adventure very quickly and easily. You will have to learn a few ins and outs of the editor, but once you figure out what does what, you are on your way. There is a guidebook out that focuses only on the editor, and I recommend getting it if you want to know more about the toolset and how it works.
The DM client. When you load up the DM client, you choose a saved module that you made, or that someone else made(But why would you wanna do that?). You load up the game, players join your server, and you basically control what happens in the game. You put down monsters, NPCs, animals, treasure, traps, etc... You can talk to people in your module(if they get lost or can't figure out what to do next) using text chat. You can even take control of any monster or NPC that you put in the game. You wanna put a scare into a few adventurers, have a dragon fly over and tease them a little bit.
The possibilities are endless with NWN, you just have to put your mind to it. NWN is by far, one of the best RPGs ever made for the PC.
So get out and buy NWN, it is worth every single penney IMO.