Home made content for games has a history almost as long and certainly as distinguished as computer games themselves. Almost any game that captures the imagination of a few hundred or so fans will spawn some sort of modding community. Players who love the game but maybe feel something could have been done differently, or want to add more content, or who want to use the engine as a base for something new will inevitably take matters into their own hands and the results can be amazing. From game changing hacks in Elite on the BBC to total conversions of the Quake engine, they all have aspects in common.
Mods tend to fall into several categories. The simplest are often those that affect the game balance, such as modifications to unit stats in a strategy game. These are accomplished often by simple changes to numbers in a relevant file somewhere.
The next type of mod is the cosmetic mod. These are usually nothing more than new skins for models or new textures for maps and areas. Depending on the standards both of the art and the implementation these can vary immensely in complexity and quality. A recent example of such a mod would be the Lara Croft lingerie mod for Tomb Raider Legends, which does precisely what it sounds like it should do.
Actually adding content to a game is the next step up. This can be simply using a supplied level editor to build maps for a game; or something more advanced such as adding extra units to a strategy game with their own graphics and animations. Beyond this some modders go so far as to build whole single player campaigns with new characters, locales and levels, for instance the Burden of 80% Proof mod for Deus Ex, in which the character must procure provisions for a weekend of drunkenness.
The 80% proof mod for Deus Ex (left) and Tomb Raider bikini mods (right).
The final step up the modding ladder is what is called a total conversion. Total conversions are very rare as they entail putting together a game almost entirely from scratch based on the engine of another game. A lot of total conversions appear, but few ever make it past the alpha stage due to the insane amount of work that is required. Total conversions done well can be brilliant games in their own right, the classic example of which being CounterStrike, one of the most successful games of all time - cooked up by enthusiasts in spare evenings.
Modifying games with player-made add-ons is best done with care. The first precaution is to keep your game disks, patches and saved games somewhere safe. Depending on how the game engine implements mods you may have to permanently change some of the game files and this means you will often need to reinstall to play the game in its original form. The second precaution is to make sure that any mods you do get are compatible with your version of the game. This is particularly true when dealing with older games - some mods may not be compatible with the latest version of the game, or else you might need to patch your game up to use the latest version for the mod to work. This relevant information in this regard should be in the mod's ReadMe file, and reading the ReadMe is always a good idea, if only to get the full explanation of exactly what your newly acquired mod will do.