Blizzard has posted an illustrated tutorial for their new Battle.net launcher, which was required to install Reaper of Souls and is quite an improvement over the old Diablo 3 launcher. (It integrates all your Blizzard games, saves you from having to log in or use your authenticator more than once a month, shows you which of your friends are online in which games, etc.)
You don’t have to use the launcher now, but you will when the Diablo 3 v2.0 patch hits, so you might as well get it now? If you’ve got it or not, when the v2 patch goes live there’s going to be a tool running to optimize and improve your Diablo 3 performance, probably by streamlining 2+ years of patches and hotfixes into one more coherent and cohesive whole.
Heads up for those who’ve already upgraded their launcher to Battle.net (or who plan to do so before Patch 2.0.1 launches):
On patch day, you will also have to go through a code-optimization process similar to what’s outlined Step 1 above. Expect it to take 15 to 60 minutes or so, and keep in mind your computer might run a little slower than usual during the process.