Interface AddOn Kit
The AddOn Kit is an executable created by Blizzard to enable AddOn authors to extract the art, XML and Lua files from the current installation of WoW so they can use them in AddOns and learn from them.
Quote From: World of Warcraft Interface AddOn Kits
World of Warcraft’s user interface is heavily customizable through player-written addons. If you want to write an addon, you should check out the addon kits below and visit our UI & Macros forum to get started.
Warning: You shouldn’t try to customize the interface unless you know XML and Lua. If you don’t thoroughly understand these tools, you run the risk of breaking your game installation.
AddOn Kit Download Links
You can download the World of Warcraft Interface AddOn Kit at the appropriate link below:
Ensure you let the Blizzard Launcher completely finish downloading/updating including any and all languages (see European Language Pack for more infomation on languages) before using the AddOn Kit.