If you're looking forward to explore the planet of Outland, or play as either of the Draenei or Blood Elves (with their respective unique class options) in World of Warcraft Classic, now's the time to get ready. The pre-patch for the first expansion to WoW Classic has now been rolled out, and if you're not planning to transfer (or clone) to a permanent Classic server, then you'll be getting the patch the next time you update.
Burning Crusade was the very first expansion for World of Warcraft, released in January 2007 and brought a lot to the game, including a new realm, dungeons, races, and weapons, among many other additions. It's now the first expansion that's being added to World of Warcraft Classic, expanding on the gameplay of that old-school version of the game and updating many of its features. If you want to continue to play pure Classic, with only the base game, there will be servers where you can do that, but for those wishing to move over to Burning Crusader, it's now rolling out and will be officially playable on June 1/2.
Cloning your character does come with a fee — it was initially priced at $35, but after complaints from players, it's now been reduced to $15. This will make a pure copy of your existing WoW Classic character, and maintain one on both a Classic only server, and another on the Burning Crusader server. From there their future's will split and you'll maintain them as separate characters, but it gives you the chance to enjoy Burning Crusader as it was when the original expansion was released, as well as continue to play through the Classic game to experience all it had to offer for early players before all of the additional content was released.