Blizzard announced earlier this month that it would offer a pre-download for the expansion. According to the post, the background download will make that patch a faster process. It's labeled as data for a "future patch" and is around 6GB in size.
"By making this data available early, you'll have less to download once the pre-expansion patch and expansion are released," the post reads. "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 3 as normal, and your in-game experience should remain unaffected by the download itself.
To work properly, Diablo 3 must be patched to the game's latest build.
Diablo 3: Reaper of Souls was announced in August 2013. The game launches March 25. Reaper of Souls will introduce the new Crusader class, increase the game's level cap to 70, introduce new enemies and more.