There are two ways, as far as I understand, to extract items from WoW. The first is illegal, and the second is as legal as can be.

First, you can open the MPQs with a program like WinMPQ (google it) and see if you can decypher how they store them. In my experience, it's a difficult process, but since every item that could ever be in the game is in your MPQ, people have found the stats of GM items and so forth using this method. However, posting anything about items you found in the MPQs on the official forums gets you banned quickly.

Secondly, you can use a LUA plugin to gather game information. This is how sites like Thottbot, Allakhazzam, etc gather their information. They have a combination of a LUA plugin that gathers information from the game and a .exe file to upload it to their sites.

The thing you would focus on is the LUA plugin. I've never bothered to learn LUA at all, but all plugins are open-source because they have to be. The best the coders can do to protect their work is to obsfucate the source code, which would be really annoying. I would suggest simply downloading the plugins from big sites (Thottbot, Allakhazzam, WoWGuru, WWNData just to get you started) and look through their source.

The only iimitation with this method is that it can only extract items that you see (linked on a chat channel, in your inventory, seen from inspecting someone, whatever), which is why major sites ask you to download a plugin to help their database. Given the massive number of items in the WoW world, even Thottbot is constantly seeing new ones when they have thousands of users.

All in all, good luck to you. Feel free to ask any questions you have.