The original version of Windows 8.1 was released in October 2013. Since then, Microsoft has released a few major updates for the Windows 8.1 platform, including Update 1 (April 2014), Update 2 (August 2014), Update 3 (November 2014) and subsequently a bunch of cumulative rollups, including security fixes and stability improvements. Windows 8.1 has exited mainstream support on January 9, 2018.
End of mainstream support refers to the date when Microsoft no longer provides non-security fixes, updates, or online technical assistance. This is the time to make sure you have the latest available update or service pack installed. When extended support ends, Windows 8.1 will no longer receive security updates that can help protect your PC from harmful viruses, spyware, and other malicious software that can steal your personal information.
This period from now until January 2023 is called as extended support.
The original release of Windows 8 was full of controversial features. The initial release had no start button on the taskbar. It was the first version of Windows with two management interfaces: the classic Control Panel and its touch friendly counterpart, PC Settings. Instead of the Start menu, there was the Start screen. These changes were not welcomed by many users.
As of this writing, Windows 8 and 8.1 versions of the OS have less than 6% of market share.