The two expansion packs for Neverwinter Nights are Shadows of Undrentide (SoU) and Hordes of the Underdark (HotU). Each expansion adds new feats, skills, spells, and prestige classes, as well as providing a new official campaign and resources (creatures, tilesets, etc.) for module builders. In addition, the HotU expansion raises the level cap from 20 to 40 (in all modules, not just ones that require HotU). The HotU expansion is the later of the expansion packs and includes the feats, skills, and spells that were added with SoU. The SoU expansion is still required to access its prestige classes and campaign.
Once installed, the expansion packs are deeply integrated into the game. One consequence is that the new feats, skills, spells, and classes are available to players in (almost) any module they play, including the original campaign (but excluding those that specifically alter the normal selection). Another consequence is that, unlike some other games, there is no such thing as patching just the original game or just an expansion pack — the original game and all installed expansion packs are always at the same version number. This integration also affects the Toolset; once an expansion pack is installed, anything saved by the Toolset will be flagged as requiring that expansion pack.
The campaigns of the expansion packs (rated at 20 hours each; the HotU campaign lacks multiplayer support) tell a linked story that is intended to be started (in SoU) with a new level 1 player character. (Part of the reason for starting over at level 1 is that SoU introduced some feats that can only be taken at level 1.) Various conversations in the campaigns indicate that the SoU campaign occurs at the same time as the original, while the HotU campaign occurs later. In particular, the HotU campaign features some non-player characters from both the original and SoU campaigns who will freely refer to events within their campaigns. These characters will furthermore assume the player's character had completed the SoU campaign and had not participated in the original (regardless of what a player might have actually done). As some of the events referred to would be considered spoilers, this leads to the interesting assumption that the player, but not the player's character, has completed the original campaign.
The SoU campaign is intended to get characters to level 12 (although ending at level 13 might be more common), while the HotU campaign is intended to be started at level 15. This leaves a gap in both the story and levels of the two campaigns. While going directly from one expansion campaign to the other is supported (the HotU campaign will accept level 12 characters; lower-level characters get promoted to 15), there was a contest held for community-created modules intended to fill this gap. The contest was neither held nor started by BioWare, but they did support it.
There have been numerous endeavors by players to expand the content in Neverwinter Nights. The Community Expansion Pack (CEP) in particular is considered by some to be a third expansion pack. There are notable differences between CEP and SoU/HotU, though. Perhaps the most noticeable is that CEP is free, while SoU and HotU are sold for profit; CEP is from the players, while SoU/HotU were created by BioWare and underwent BioWare's quality control. In addition, SoU and HotU are integrated into the game, while CEP exists as hak paks. Another notable difference is that CEP focuses on new resources for builders, rather than resources for players (such as spells, classes, and campaigns).