Realmlist.wtf

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. After patch 3.0.2, the file was moved out of the main World of Warcraft folder into a subfolder (\World of Warcraft\Data\enUS\ or another language). After patch 5.0.4, the file was removed entirely.

File contents
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:

For US clients:

Patchlist directive
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.