This is the latest version of Windows - it's a media creation tool that can be used to upgrade an existing installation of Windows or create installation media for another PC. Just make your choice after launching the tool and it'll do the rest.
What's new in Windows 10? The latest release brings back the Start Menu, though with a Windows 8 twist: live tiles keep you up-to-date with the latest news while also providing an easy way to launch apps. (Don't worry if you prefer the Start Screen, it's still there and you can boot into it if you prefer.)
Apps now work much more like regular desktop programs. They have minimise, maximise, restore and close buttons, and can be resized (to a degree) and organised however you like.
If you've still lost track of a program in the mass of open windows, a new Task Spaces feature can help. Click its taskbar button and you'll see thumbnails for everything running now, a little like OS X's Mission Control - just click something to switch to it.
Better still, Task Spaces also supports virtual desktops. Add extra desktops as required and it'll display thumbnails of each one, making it easy to identify whatever you're after and switch to it.
There's also smarter snapping, new customisation options, and even a bunch of experimental additions to the command prompt.
Windows 10 is offered as a free upgrade to Windows 7 and 8/8.1 users in its first year - just run the tool to upgrade your existing version, but we'd first recommend you take a fail-safe backup, just in case - try EaseUS System GoBack Free if you don't have a drive-imaging tool to hand.
Note: this will install the 64-bit version of Windows 10.
It's a natural evolution for both Windows 7 and 8.1 users, bringing back the Start menu for the latter while adding useful new tools like Task Spaces, Cortana and app windowing.