What is a FOMOD file?
Game files used by Fallout mod programs such as Fallout mod manager (FOMM) and Nexus Mod Manager (NMM); contains files that, when installed, modify game content for Fallout 3 or Fallout: New Vegas; used to apply custom mods developed by members of the gaming community.
The FOMOD format can use .7Z, .ZIP, or .RAR compression. Inside the compressed archive, there is an info.xml file that describes the mod and the script that installs the mod. There are also data files that change in-game content.
FOMOD files can be installed by double-clicking while FOMM or NMM is installed. FOMM opens and displays mods in the content window, and you activate it to apply mods. Mods used by FOMM are generally installed to the Fallout 3\fomm\mods\ folder in the game’s installation directory.