Microsoft Windows (or simply Windows) is a metafamily of graphical operating systems developed, marketed, and sold by Microsoft. It consists of several families of operating systems, each of which cater to a certain sector of the computing industry.

Windows 10
The first year after Windows 10 launches, any device running Windows 7, Windows 8.1, or Windows Phone 8.1 will be able to upgrade to the latest version of MIcrosoft’s OS — for free.

Starting from descending order to explain the history of Windows Operating system:
Windows 10: Key features of Microsoft’s new operating system
Microsoft Windows 8.1

The product falls under the same lifecycle policy as Windows 8 with support ending 1/10/2023
Windows 7
Microsoft reportedly end the support of Windows 7 in 2015

Windows Vista
Microsoft will end “Mainstream Support” for Windows Vista, moving into an “Extended Support” phase that lasts through April 11, 2017.

Windows XP
Microsoft ended Official Windows XP Support in April 2014