Microsoft's new scripting shell, formerly known as Monad or MSH
PowerShell 2.0 is installed by default on Windows 7 and Server 2008 R2, and can be downloaded for Windows XP SP3, Windows Server 2003 SP2, Windows Vista SP1 and Windows Server 2008 SP2.
PowerShell 4.0 is installed by default on Windows 8.1, and can be downloaded for Windows 7 and Windows 8.
PowerShell 5.1 is installed by default on Windows 10.
PowerShell 6.0 can also be installed on Linux and MacOS.
For PowerShell 4.0 on Windows 7 and 8, Microsoft .NET Framework 4.5 is required.
PowerShell won't run on Windows Server 2008 Core, as the Windows Server 2008 Core configuration doesn't support any .NET Framework version.
The interpreter is the "engine" that executes scripts.
It is installed by default on Windows 7 and Server 2008 R2 and later.
It can be downloaded for Windows XP SP3, Windows Server 2003 SP2, Windows Vista SP1 and Windows Server 2008 SP2 (you may need to uninstall older versions of PowerShell first).
To use PowerShell on Linux or MacOS it must be downloaded and installed first.
If you want to create and run your own PowerShell scripts on Windows, you need to digitally sign your scripts.
To create your own so called "self-signed" certificate you'll need makecert.exe that comes with the .NET Framework 2.0 SDK.
To learn more about signing your scripts, start a PowerShell shell, type Get-Help about_signing and read this text carefully.
Then type Get-Help Set-AuthenticodeSignature -detailed and read this text just as carefully.
In PowerShell Plus creating a "self-signed" certificate takes only a couple of mouse clicks.
Most up-to-date, right now, are Microsoft's PowerShell pages on MSDN and TechNet.
Much of the help available on the web applies to PowerShell 1.0, but it is still valid.
Read the Windows PowerShell Owner's Manual.
Other help on the web: