Microsoft pushes out critical updates to fix security flaws and issues with the operating system, but usually on “Patch Tuesday”. In a somewhat unusual move, Microsoft is pushing out over 30 updates for Windows 7, Windows 8.1, Windows Server and even Windows RT today, in an unscheduled release. These are all optional updates on top of some of the unspecified security updates.
As far as the changes are concerned, the official knowledgebase article mentions a lot of changes which will come with the update, fixing a wide range of issues for the above mentioned operating systems.
You should start getting the update as we speak if you have automatic updates turned on. It will download the updates, and do keep in mind that most of the updates require users to restart their computer after the installation of each one of them. Let us know using the comments below if you installed the updates.