Along with the usual “Patch Tuesday” updates last week (8th April) Microsoft also released the following updates:
Windows Server 2012 R2 Update and Windows 8.1 Update (KB2919355)
These are actually a cumulative update rollup and contain some new features.
Windows 8.1 features are detailed here http://windows.microsoft.com/en-gb/windows-8/whats-new
These updates should be installed to ensure future patches are received, see this from Microsoft –
Important All future security and nonsecurity updates for Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2 require this update to be installed. We recommend that you install this update on your Windows RT 8.1, Windows 8.1, or Windows Server 2012 R2-based computer in order to receive continued future updates.