Developers such as id Software, Valve Software, Bethesda Softworks, Firaxis, Crytek, The Creative Assembly and Epic Games provide extensive tools and documentation to assist mod makers, leveraging the potential success brought in by a popular mod like Counter-Strike. The Internet provides an inexpensive medium to promote and distribute mods, and they have become an increasingly important factor in the commercial success of some games. These mods can add extra replay value and interest.
Games running on a PC are often designed with change in mind, allowing modern computer games to be modified by gamers without much difficulty. Mods that add new content to the underlying game are often called partial conversions, while mods that create an entirely new game are called total conversions and mods that fix bugs only are called unofficial patches. They can be single-player or multiplayer. They also usually take place in unique locations.
They can include new items, weapons, characters, enemies, models, textures, levels, story lines, music, and game modes. Mods are made by the general public or a developer and can be entirely new games in themselves, but mods are not stand-alone software and require the user to have the original release in order to run.
Mod or modification is a term generally applied to personal computer games (PC games), especially first-person shooters, role-playing games and real-time strategy games.