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\).
As of the latest version 3.x.x patches, the realmlist has had the set realmlistbn "" line added for the new battlenet system. enGB client used as example:
set realmlist eu.logon.worldofwarcraft.com set patchlist enGB.patch.battle.net:1119/patch set realmlistbn "" set portal eu
For US clients:
set realmlist us.logon.worldofwarcraft.com set patchlist enUS.patch.battle.net set realmlistbn "" set portal us
Around patch 2.1, a new directive was added to the to the realmlist.wtf, the current use of this directive is unknown, but it's believed to be added to support the Blizzard Launcher's latest feature to notify you of new patches, the launcher itself connects to us.version.worldofwarcraft.com and gets a file called PatchSequenceFile.txt. This file contains an update path for the game, and the launcher.
Post-patch 2.1, the patchlist directive now also supports the Public Test Realm Clients, and updates those as nessecary, as such the file now have additional contents. There is also support for the background downloader although it is hard to say when this was added.