An add-on is often considered an extension that can be used to modify the behavior of existing features to the application or add entirely new features.
A mod or modification is a term generally applied to PC games. Mods are made by the general public or a developer, and can be entirely new games in themselves, but mods are not standalone software and require the user to have the original release in order to run. They can include new items, weapons, characters, enemies, models, textures, levels, story lines, music, and game modes. They also usually take place in unique locations. They can be single-player or multiplayer. 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 are called unofficial patches.
The following is a list of modifications and add-ons for Quake 1. All files have been repacked to either include the latest patch, or for better compression. Their behaviour has not been modified, and original properties have been retained.