When you activate Diablo 3 launcher or the Battlenet Desk app, you would find that it would down load a patch automatically. This is the preparation for the Reaper of Souls which would come online on 2014/03/25 and the Prepatch.
How does it work:
This patch is around 6GB including the picture files, 2.0.1 patch and some complete content of expansion. The earlier you download, the less you have to download when these content come online.
The files you are downloading is encrypted. It will not affect your game and server until Patch 2.0.1 and Reaper of Souls. You can play your Diablo 3 while downloading the patch without any influence.
To activate the download, you have to download the Diablo 3 latest edition fist. Even if you have not pre-purchase the expansion, the content can be downloaded as well.
If you are using Battlenet Desk app, it would download as soon as you have activate the app. You can pause or continue just by the click of the right bar. You can set your own download by limit the downloading speed by the Set icon.