To make a game, bethesda uses 3 types of files. .bsa, .esm, and .esp. The game's .exe merely knows how to start and run the information in those files (plus scripts and other stuff, but whatever). .bsa - Bethesda Archive, these are archives similar to .7z archives, but less compressed and easier for a program to get into quickly. These archives contain the textures, meshes, sounds, and anything else that your monitor would show you or that your speakers would say to you. To modify appearances and sounds one would make a file in the folder that corrosponds to the filename and directory in the archive. For instance, if you wanted to make a replacement texture for female glass armor (there's another texture for male armor) you would open the .bsa and find that the file in question is /textures/armor/glass/f/curiass.dds (direct draw surface, many image editors like paint.net and gimp can edit these). One would make the texture look how they wanted it to look, then create the directory /textures/armor/glass/f in the data folder (if it didn't exist already) and place the a file named curiass.dds there. If the game finds a file with the appropriate name in the appropriate location, it will load that instead of loading the image from the .bsa, thus the data folder is its own override folder. You can CREATE your own .bsa archives if you want to be neat and clean, but you can't override the images of other .bsa doing that without using .esp. .esp and .esm - Elder Scrolls Plugin and Elder Scrolls Master, despite what the name suggests these are not limited to the elder scrolls series, they also serve the same purpose for Fallout 3 and Fallout NV. The .esm will most likely not be touched by the modder, and I'm actually not certain what the difference is between the master and the plugin. These files contain large ammounts of instruction that tell the game what to do, these are the files that hold information such as weapon damage, weight, and how annoying it is to be overburdened. One can modify a .esp to make the game do anything, mostly by telling it which scripts, textures, meshes, and animations to load and when. One does not modify any of the existing .esm or .esp files, but instead makes new ones. If you make a brand new .esp, you can copy existing records in other .esp or .esm files and change values and strings, making sure not to change the reference, so that game still calls it the same part of the game, you would then place this new .esp lower in the load order than the one that contained the object changed. When the game starts it loads each .esm and .esp one by one, whenever it finds the same record in a .esp or .esm that has already been loaded, it unloads the old record and replaces it with the new. Thus you have nice, neat, modular plugins. Further and grander things like scripting are beyond me. TL:DR http://www.nexusmods.com/skyrim/mods/modmanager/?