Diablo 3: Reaper of Souls is scheduled to release on March 25, but Blizzard Entertainment announced that you can go ahead and download the background right now. This means that the game launcher and Battle.net desktop app will begin downloading data for a “future patch.”
The following diablo III description is taken directly from Battle.net:
“The download will be around 6GB in size and include things like models, textures, art files, and other upcoming content in patch 2.0.1 and Reaper of Souls that is already complete. By making this data available early, you’ll have leass to download once the pre-expansion patch and expansion are released.
The background download data is encrypted and won’t make any changes to the live game or Battle.net service until patch 2.0.1 and Reaper of Souls launch. In the meantime, you will be able play Diablo III as normal, and your in-game experience should remain unaffected by the download itself.
In order for the background download to begin, Diablo III must be installed on your computer and patched up to the latest build. Please note that you will be eligible for the background download even if you have not prepurchased Reaper of Souls.”