Neverwinter Script is a C based programming language that is used in Neverwinter Nights to make all sorts of fun things happen.
The main strength of Neverwinter Nights is its toolset. Neverwinter is one of the first RPGs to allow users to (relatively) easily create their own adventures or "modules". NWscript is the business end of the toolset. It is used for everything from monster AI, event-handling to running convesations between the Player and NPCs.
Since it's based largely on C, it isn't overly difficult to get working, especially if you have prior coding experience. Scripting tutorials can be found at Neverwinter Vault (http://nwvault.ign.com/) or at the Bioware Community site (http://nwn.bioware.com/, in the "builders" section), where I'm a moderator.