An area is a single setting within the NWN2 game. In simplest terms, it is a space wherein the PC can move and act without going through a transition.
To a certain extent, it is possible to mix different tilesets to create a single area. However, they have to be separated by doorways, walls or other strong borders defined by their own tileset specific properties.
Using the analogy that playing NWN is like reading a story, if the module is the book, the areas are the chapters.
NWScript[]
An area is an object and can be targeted within a script using GetObjectByTag or GetArea.