The Garden of Eden Creation Kit (GECK) allows you to edit and create any data for use with Fallout 3. The GECK is used to create game content for Fallout 3. The data is stored in data files that are read by the game. The GECK allows you to create your own areas (towns, dungeons, etc.) and populate them with your own characters, creatures, items, and stories.
For more information on how to use the G.E.C.K. head to the G.E.C.K. wiki: Geck.bethsoft.com