Editing WoW:User interface customization guide (section)

== Lua scripting ==
All of the functionality in the interface is provided through Lua scripting. It is also possible to create any layout that is possible with the XML markup in pure Lua scripting.

The manual for Lua 5.1.4 (http://www.lua.org) is available online at http://www.lua.org/manual/5.1.

For more concise help, topic-specific Lua tutorials can be found at: http://lua-users.org/wiki/TutorialDirectory.

The best way to become familiar with the way Lua is used to script the interface is to look at the scripts in the XML files, denoted by the &lt;script&gt; tag, and to browse the lua files. The lua files typically contain functions which are used by the corresponding XML files.

As a reference, the [[World of Warcraft API]] page contains a (almost) complete list of available API functions in ''World of Warcraft''.   When you want to figure something out, this is the first page you should turn to.  Also the [[Widget API]] page contains an overview of the methods that are available when dealing with objects of the user interface — like action buttons or unit frames. Feel free to play around with the <tt>print()</tt> function to try out the various API functions.

For example type this into the game chat:
 /run print(GetLocale());
GetLocale() is an API function.  It returns the client locale  ("enUS" for English clients, "deDE" for German clients, etc).  Print() then prints that value into the default chat window.  
Your addons are loaded after the Blizzard files, so your addon can overwrite or change any Blizzard UI.  For example, you could create an addon with one line of code:
 MailFrame:Show()
MailFrame is the name of a Blizzard Frame, created in Blizzard Interface Data/FrameXML/MailFrame.xml.  Anyframe:Show() will make that frame appear, even if you are nowhere near a mailbox.  Type /run MailFrame:Show() in game to accomplish the same thing.