Fallout File Types
The engine used in Fallout 3 and Fallout NV are built on the engines used by Oblivion and the Elder Scrolls games. The file names reflect that history and their extensions have not changed in name since the first engine was built. There is quite a bit more information on these topics to be found here. To recap, though, there are some file types you'll likey come across in your modding forrays.
- .esm
This file extension stands for Elder Scrolls Master. This is the root file and usually is whichever is provided by the game; such as FalloutNV.esm. - .esp
This file extension stands for Elder Scrolls Plug-in. Most of your extensions will be of this file type. - .nif
These are usually found in the Data/Meshes directory. They describe the objects you'll find in the game. NIF is short for NetImmerse File. - .bsa
BSA files are an archive format that contains many of the resources, in most-cases, used by the .esp files you or others have written. They are less easily modifiable but are convenient when you're ready to start shipping your product.