Post by Miyari
Not only do we have an excellent local Interfaces & Macros FAQ
which provide great resources and information, but Cairenn
has even more information regarding addons and other UI aspects (such as legality, as you so questioned) covered in-depth in her Mods FAQ (AddOns, UIs, Interfaces)
thread on the official US WoW forums.
To quote a forumgoer, Trimble, on this specific topic (as found in section 5 of her FAQ):
UI AddOns fall into two categories:
Macros - these aren't really AddOns at all, but they can use LUA code to do neat things in game. These are found ENTIRELY inside the game, and the code is run by Blizzard's interpreter, so they make the rules on what macros can and cannot do. Therefore, Blizzard won't ban someone from using any macros.
AddOns - These are extra files, put into the WoW Interface directory on your hard drive which supplement the existing LUA and XML code that Blizzard created that makes up the user interface that everyone uses.
AddOns are made up of LUA files, and XML files, both of which are also run by Blizzard's interpreter. Therefore, Blizzard won't ban someone from using any AddOns.
3rd party programs are actual programs that get loaded into memory and executed using their own code. They run separate from the game similar to how ICQ or MSN Messenger runs in the background. The difference is that 3rd party hacks try to read, or change data coming into or out of the WOW game client(to the network card), or they try to read or write to memory space used by the Wow Game client.
Speed hacks, for example, often interfere with data flowing from the game client to the network card by intercepting the "i am here" signal that he game client sends to the server and making it seem that the player is moving faster than they should be. The server accepts this as truth because Blizzard left it up to the game client to limit how fast you can go. (Oops Blizzard?) This strange design is caused by a need to let the game client do some of the work because the server is too busy to be expected to do everything.'
Anyway, the distinction in 3rd party hacks is usually the fact that hacks run as their own programs and interfere with the game client. AddOns aren't anything more than REALLY extravagant macros.
(Note: Cosmos is known to have an external EXE file that gets run as it's own code - however, it doesn't interfere with the game client - in fact, all it does is download LUA and XML files, puts them in the right places, then QUITS long before the WoW game client even starts.)
And the official blue post by Slouken to quickly summarize the wall-of-text:
This is a very good explanation of things as I understand them. However, I am not involved in policy and I'm not a lawyer.
If you haven't been explicitly warned by a GM, or seen an official Blizzard response about an AddOn or UI modification, then don't worry about it.
I recommend checking our her post, as well as WoWInterface
for addons and UI files. :)