File Formats

2DA: Two dimensionnal arrays
BIC: Saved Character Data
DDS: 2D image compressed at mutiple resolutions, used for textures
ERF: Exported extra content, for builders to import it back into their modules.
HAK: Resource Packs - see hakpaks.
INI: Settings and Configuration file (areag.ini)
IFO: Module Information File
ITP: Palettes
MDL: Models - see Modeling
MOD: Module
NWM: Neverwtiner Nights Campaign Module
PLT: Palleted Texture - see textures
SET: Tilesets definitions
TGA: 2D image, used for compatability and low resolution textures, including GUI components which don't require the use of PLT's.
TXI: Custom Texture settings
UTI, UTC, UTT, UTE, UTW, ITP, IFO: Blueprints. UTC creatures, UTD doors, UTE encounters, UTI items, UTM merchants, UTS sounds, UTT triggers, UTW waypoints. Which can be viewed with ITP tools (http://nwvault.ign.com/Files/other/data/1029208648677.shtml)
WOK: Walkmesh data for tilesets, and occasionally placables