what is Windows s

 Windows 10 S is streamlined for security and performance, and works exclusively with apps from the Windows Store

Microsoft has yet to reveal what Windows 10 S stands for, but ‘Speed’ would be as good a choice as any.
if you’re setting up multiple computers, Windows 10 S can also be installed via USB with preconfigured options. That’s a big time saver.
Windows 10 S will boot in just 15 seconds. It is built on the same core as Windows 10 but is more lightweight, running on lesser hardware configurations which allows for both the purchase of very cheap PCs (more later) but also installation on older hardware to bring it back to life.

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Cloud 

Windows 10 S integrates with OneDrive so files are saved to the cloud, in sync and accessible from your devices. Use compatible apps available from the Windows Store. If you need to use professional tools or would rather run non-Store apps, you can easily and affordably switch to Windows 10 Pro

Download and read more
https://www.microsoft.com/en-us/windows/windows-10-s

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