The New World of Warcraft Launcher allows you to stream game data while you play. This means less time waiting for patches, and more time playing.
There are three stages of download progress:
SetupCore game content is being prepared. You cannot enter the game at this time.
PlayableMajor game content is not completely applied. You can play, however your game experience will not be ideal.
OptimalFinal game content is almost complete. You may experience a few game issues while playing at this stage.