Microsoft released Windows 8.1 in October 2013, and while it was recommended for all Windows 8 users, it was not a requirement to continue receiving support from the company. That won't be the case for Windows 8,1 users when it comes to Tuesday's release of Windows 8.1 Update as it will be delivered via Windows update.
In a Microsoft Premier Field Engineering Platform blog post, the company flat out says that Windows 8.1 Update is a required download for Windows 8.1 users. The blog post states:
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 yet to update to Windows 8.1 don't necessarily need to also download this update, although it might not be a bad idea. Microsoft has already announced that it will stop supporting Windows 8 in the very near future. Previously, the company indicated that it will end support for Windows 8 two years after the launch of Windows 8.1, which would be on October 18th, 2015. Now a new FAQ page says that the final date for Windows 8 support has been moved back a few more months and it will now end on January 12th, 2016.
Mainstream support for Windows 8.1 will end January 9th, 2018 and Extended Support will stop on January 10th, 2023.