Today's Patch Tuesday is special because of a couple of reasons. First, it is the last patch public patch day for most Windows XP systems. I do not think there is need to panic just yet, but you may want to consider upgrading the operating system eventually.
Second, Windows 8.1 Update will be released today. Microsoft revealed the update officially some time ago, and we have reviewed what is new last month as well.
It improves accessibility of the operating system for mouse and keyboard users, especially on the Start Screen side of things.
While that won't make the Start Screen tolerable to users who dislike it, it improves its usability for users who use it at times or regularly.
To name the most important additions: Apps now display a title bar that you can use to close or minimize them. There is a new right-click menu when you click on app tiles, and a new shutdown button on the start interface. Non-tablet systems will also boot to desktop by default now, and media files open in desktop applications if available.
Today may also be the day the new Windows Store update rolls out, but Microsoft has not confirmed nor denied that yet.
The update is mandatory
So why is that update mandatory? The reason is simply: If you are running Windows 8.1, you need to update to Windows 8.1 Update to receive future system updates.
That's right. If you do not update, you won't receive any patches for the system on the next patch day or any other day until you update to this release first.
This is confirmed by Microsoft employee Michael Hildebrand on this blog post.
It is a required update to keep your Windows 8.1 device current
Failure to install this Update will prevent Windows Update from patching your system with any future updates starting with Updates released in May 2014 (get busy!)
Windows 8 users who have not updated their systems to Windows 8.1 yet on the other hand are not affected by this rule. They continue to receive updates for their version of the operating system even if they do not upgrade to 8.1 or the new update that is released today.
As far as prerequisites go: Windows 8.1 is a prerequisite which you can download from the store or if you have access to Windows 8.1 ISO images using them as well.
Another prerequisite is KB2919442 which you can download directly from the linked page or via Windows Update.
Windows 8.1 Update is a cumulative update that contains all previously released security and non-security updates. The update itself is defined as Important - Security Update which means that it will be downloaded automatically on most systems, unless Windows Update has been customized.