Fallout 3 (and previous versions of Fallout) store game data in ESM files. These files contain the maps, objects, dialogs, and artificial intelligence (AI) settings used by the Fallout games.
ESM files are referenced by Fallout 3 and are not meant to be opened manually. However, they can be altered or "modded" by a ESM-editing utility, such as The GECK (The Garden of Eden Creation Kit).
Fallout 3 is the third game in the series of Fallout video games. It is a single player role-playing game that places players in a post-nuclear fallout environment known as "Capital Wasteland." Players navigate their way through the 3D environment in order to accomplish the game objectives.
ESM stands for "Elder Scrolls Master," and is the primary data format used by The Elder Scrolls IV: Oblivion. It contains several types of game data, including audio, video, 3D models, textures, and game menus.
ESM files are referenced by the game, but can also be opened and edited using an ESM editor, such as The Elder Scrolls Construction Set. Custom ESM files can be used to "mod" or alter the gameplay of Oblivion.
