Background download of the next patch will start to deliver content in preparation of the Burning Crusade expansion. Update your network card drivers if you suffer excessive lag. If you prefer to disable the background download due to lag or other issues, you can start the launcher and choose options. The last option disables it. Source: Wow Forums:
Drysc: Later today we will begin providing most of the remaining data from our upcoming patch for players to background download. This second phase will be very large (approximately 450MB) as it contains the data for all of the upcoming changes as well as art assets needed in preparation for the release of the expansion.
This data will be delivered at a very slow rate in order to reduce noticeable affects on the performance of your PC. By default, the background downloader will launch after game exit, and will stop upon game re-launch. However, you may wish to take advantage of one of the other background download options.
For more information, read the Background Download FAQ, found here: http://forums.worldofwarcraft.com/thread.html?topicId=48524887&sid=1