Mod

Mod is short for "Modification." A mod is an after-market, player created AddOn that alters or enhances some aspect of WoW's interface, usually improving gameplay efficiency and efficacy without actually altering the gameplay itself.

Some AddOns change one or several default UI parameters. This could be something as simple as making one's character information displays moveable. It could be adding an extra button bar so one doesn't have to hit so many keystrokes to access frequently used skills, abilities or spells.

By the same token, some are as complex as to completely replace the default UI with another, offering in most cases, an increased functionality set.

NOTE: For the sake of clarity, I (Arath) will use the term mod to mean a complete UI replacement and the term AddOns to denote a module that can be plugged into one's existing UI no matter if that UI is the default one or yet another mod.