What are AddOns? How do I install AddOns?
AddOns are Lua/XML files that can modify the WoW User Interface. You can install an addon by extracting its folder(s) into the addons directory:

World of Warcraft\_retail_\Interface\AddOns

How do I develop AddOns? Where can I get AddOns?
Start with Create a WoW AddOn in under 15 Minutes. They can be downloaded from CurseForge and WoWInterface or one of the addon updater clients.

Interface documentation

Interface customization


Things you can help out with