Blizzard announced that it will be releasing Diablo III Expansion: Reaper of Souls on March 25, 2014.
On battle.net, they have said, “As of this week, you may notice that the Diablo III game launcher and Battle.net desktop app will begin downloading data for a “future patch.” This download is in preparation for the March 25 launch of Reaper of Souls, as well as a pre-expansion patch that will be released in the coming months.”
The idea is that you’ll have less to download when the expansion is actually released. The files won’t affect or change anything until then, though.
Gamers who wish to pre-purchase the expansion can do so here.