Neverwinter Nights is a 2001 game made by Bioware. Neverwinter is a city in the Forgotten Realms setting (Elminster ring a bell? Drow? Baldur's Gate? Icewind Dale?)...

The game was made as an early 3D follow-up to Baldur's Gate, though in a different part of the world and with different characters. It wasn't as well received as Baldur's Gate because of the somewhat shallow party characters, but spawned one of the earliest major modding (modification) communities because of it's modding toolset - the Aurora Engine.

This is not to be confused with the Skyrim toolset, where you can add things to Skyrim, or the cosmetic mods used to make games like Dragon Age look better. The Aurora Toolset was more like the RPG-Maker program inspired by Final Fantasy...only in early 3D/better textures/animations...

The modifications to the game generally fall into one of these categories:

1) Hakpacks - New effects, new spell animations, Kissing animations, New Monster models made in 360smax, different settings, new character customization options - heads, hair, armor - adding exotic race rules and characteristics...playing around with the source code made somewhat easier...
2) Dialogue Trees and Sound/Music - 200 songs I have come from either this game, or from those who added their favorite music to the game (Morrowind theme in Neverwinter? Yes!)
3) Modules - These are the best things. No, you don't add a new area to Skyrim - instead you make your own world. It doesn't have to be Forgotten Realms...in fact, several of the best are in their own world. When multiplayer was supported, this allowed for Persistent worlds, free-to-play games that were almost as developed as modern MMORPG's. So what if Neverwinter Nights Campaigns didn't have the meat of Baldur's Gate? A module could easily do as well. If anyone asks where I was - why I didn't join Forums when they were popular, or why I know little about MMORPG's and all...I was busy drinking a concoction that made my head turn into a chicken in ACME Vamp. vs. Werewolf, or escaping the prison of the king, or fighting a God who enslaved heroes as characters so he could write fantasy novels...or redeeming Aribeth and not condemning her to the 8th layer of Hell for a crisis of faith...
4) Tools - many of the tools to modify things exist beyond the Aurora Toolset. There's Leto, that allows you to modify your character's appearance beyond the normal scope. There's the Reliable Henchmen mod, that turns your characters into companions you can take between games. There's the Community Expansion Pack that bundles up hundreds of the best of the Hakpacks, and doesn't create errors. There's another Community pack that adds 5-10 more classes (on top of 10+ classes already present). There's a music converter to make your music usable in the game...etc.

Yeah...I get carried away when it comes to NWN...to me, its (free) community and stories and continued new innovative mechanics were... <3

In addition, not all of the story/character material made by Bioware was sub-par in comparison to Baldur's Gate. They released a series of "Premium Modules" that were amazing. Imagine if someone made the Dragon Age mechanics allow you to ride a horse and Joust. Or get to be a pirate! Or a King of a Customizable Keep (Why does that sound familiar...?) Or a mystery of the Afterlife? Or in a 1800's-styled magic world where an opressive race periodically wipes out human civilization to keep humans from leaving their planet?...

Neverwinter Nights 2...had Bioware sell its licence to Obsidian, who made better campaigns by far (I swear to you, several of the awesome things done in Dragon Age: Inquisition were part of the NWN 2 Original Campaign...just very few romance options and worse textures...) and this is where Mask of the Betrayer came from!
...However, though several tools were also made for NWN 2, I think the issues with making and placing objects, as well as graphics trouble, discouraged people from making near as many good stories in NWN 2.