What are the differences between NWN and the expansion packs?
Like are there new levels, new playable characters and new items or what.

There are 5 new prestige classes that can be added onto an existing character. There is an entire new campaign included, which is a vast improvement over the one in the original NWN, albeit shorter. There are new spells, new feats, new items, and some minor enhancements to the game play like being able to access henchman inventories.

Also, if you dont have SoU you cant get into the IW NWN server, which runs one of the better modules and seems to attract a lot of good players as well, which is always fun. It is a high level game though, a level 12 character will get his butt kicked in short order if you get seperated from the party.