A modification is an alteration to a a game. Grand Theft Auto modifications are created by fans after the game's release, and are not authorised or endorsed by Rockstar Games. Modifications can be obtained from many Grand Theft Auto websites and fansites.
Almost all modifications are entirely free, and may be downloaded and used by anybody. However, there are restrictions on editing and distributing the modification, so always ask permission from the author.
The GTA Place is a good place to start looking at what types of mods are available. See Different Types of Modifications for examples of some of the things mods can do. Also, be sure to read the Disclaimer.
There are many tutorials for modifying simple aspects of the game, ranging from altering the colour/handling of a vehicle, to editing textures. These are simple once you understand them, and can easily be fixed if you make sure you backup what you change.
However, creating more advanced modifications (such as entire buildings or cars) is trickier, and usually requires some advance experience in creating models for games. Google SketchUp is a good program for creating building models. 3DS Max, Lightwave and Zmodeller are also used.
It is strongly advised that you do not attempt to do more than you are capable of. Do not expect to be able to recreate your favourite car in a few hours, many modifications take months.
Different Types of Modifications
Some modifications are very simple and superficial, such as changing clothing on the player, or the colours of cars. Some modification can make playing the game easier, such as increasing the speed of vehicles or the power of weapons.
Most modifications change the look of the game. These can range from simple texture edits of buildings, characters or vehicles; to replacing buildings, characters or vehicles with entirely new ones. The market for car mods is massive, and there are many download databases with hundreds of GTA car mods.
Other modifications can help the player with the game. This could include editing the data and statistics in-game, which would allow the player to run faster, reload less, jump higher, live longer etc.
An alternative is a Trainer, which is a program that runs alongside the GTA game and manipulates the game's data stored in memory. These programs can change many different things, ranging from the player's health, armour, weapons, skill and even location; to being able to spawn vehicles, modify the current vehicle (eg colour, damage etc) and edit vehicles stored in garages; to modifying the time, game speed and weather. Some trainers include keyboard shortcuts to allow the player to quickly activate (or deactivate) a feature whilst playing the game.
There are many modifications that alter the mission script (main.scm). These could alter details of missions or even add entirely new missions. The main.scm file is responsible for what-happens-when, so modifying it can achieve almost any effect. However, samegame files only work for one main.scm file - if it is altered, no existing savegame will work, and specific savegames created by a game with that main.scm version must be used.
Some modifications encompass many of these features at once - these are known as Total Conversions (TCs). TCs generally change everything in that GTA game, including the map, vehicles, pedestrians, players and missions. A few TC mods have been made, and there have been many more planned that have never reached a release. GTA London (not to be confused with London 1961 or 1969) was a hotly-anticipated Total Conversion for GTA Vice City. However, after a couple of packages of small-scale modifications were released, and the declining popularity of GTAVC, the modification was discontinued. Now, GTA Great Britain is the one to watch.
Some gamers consider modifications to spoil the integrity and theme of the game - for example, they don't think a 2005 Lamborghini belongs in a fictitious 1990s world, although you can make your own decisions on these matters.
Using modifications does not alter the status of your game like using Cheats can do. However, if you install a modification that edits or replaces the main.scm file, you will have to begin a new game, or use a provided savegame created for that specific modification.
You modify your game at your own risk. There is no guarantee that all modifications you download will work, or whether the instructions will be complete. You should make yourself familiar with simple well-known modifications (such as cars) before attempting to install larger and more complex models.
Always read the instructions thoroughly, and always backup files before you edit