- See WoW AddOn for the World of Warcraft AddOn reference.
AddOns are generally self-contained components that add to, or change the behavior of a program. Some AddOns can be dependent on other AddOns to work, and some AddOns can be just collections of other AddOns.
An AddOn for a game is often a collection files you can put in your game folder that can enhance interaction with the game.
A mod, or modification, is a third-party AddOn that alters or enhances some aspect of the program's interface or otherwise. Mods can perform a wide variety of tasks, including adding new buttons to the screen, altering or adding windows, replacing or adding significant functionality, or adding whole new programs.
Some mods are standalone, able to run without any additional software. Others are built on libraries. For example in WoW, the old Sea or Ace libraries. Others such as the old WoW Cosmos addon, are compilations of many individually-developed mods or addons.