Windows 10 S is a restricted version of Windows that uses the Microsoft Store. This version is intended to compete with Chromebooks etc. in the lower price ranges for laptop and tablet machines.
For those who owned one of the original Surface tablets, which used the ARM processor, the Windows 10 S will feel the same.
Windows 10 S will only run verified store apps. Windows 10 S also is restricted to the Edge browser. Bing is the search engine.
Microsoft will allow Windows 10 S users to unlock their systems for a modest fee. Upgrading to Windows 10 Professional will cost $50.
Windows 10 S is expected to be available for the fall of 2018.