Realmlist.wtf is a Warcraft Text File that tells the WoW client which server to connect to. As of patch 2.1, a patchlist directive was added, presumably in conjunction with the new features of the World of Warcraft Launcher. During patch 3.0.2, the file was moved out of the main World of Warcraft folder into a subfolder (\World of Warcraft\Data\enUS\). Between 3.0.2 and 3.3.5 these values were moved out of this file and into Config.wtf, this file is depreciated and these values should be there instead. This file will not be recreated if deleted, instead these values will be merged with Config.wtf, if deleted.
However since this file is still loaded and useful, documentation is provided.
The example below would be the contents from a enUS client.
set realmlist us.logon.worldofwarcraft.com -- Sets realmlist to a hostname or IP address, the realmlist is the logonserver the client logs into and receives realm data from set patchlist us.version.worldofwarcraft.com -- The patchlist is a URL that the client downloads the latest patch information from, which is a template of architecture, OS name, and game name set realmlistbn "" -- Realmlist for battle.net, this seems unused set portal us -- Battle.net portal to login to if using a email format username
It is recommended to clear the patchlist value as to not let the client patch! Also you may delete the launcher and launch warcraft directly from "wow.exe"