Microsoft is allowing the public to preview and provide feedback on their next OS release, Windows 10. Dubbed a Technical Preview, it is available to download right now at http://preview.windows.com. To download the ISO fles, you the link is located here.
Now, it's not wise to install this Technical Preview on a machine that you rely on daily. There are already a few people on Microsoft's forum who are angry there's no way to "uninstall" Windows 10 like a normal application. Unfortunately, Windows 10 is an OS, not an application. As such, there is no "uninstall". You will have to backup your data and reinstall your version of Windows to get it back. If you MUST install it on your physical machine, you should at least make a disk image using a utility such as Norton Ghost or Acronis TrueImage. They aren't free, but there are free alternative disk image utilties out there you can use instead.
There's also partitioning your disk and dual boot, but if you already thought Windows 10 could be tried out like an application then uninstalled, this is also a complicated avenue.
Rather than going through all that trouble, you can use a virtual machine instead. This way, you won't have to mess up your current installation of Windows. The virtual machine is an application you can uninstall, and the Windows 10 Technical Preview just runs inside it. This is the easiest way for non-technical people to try out the Technical Preview or any other OS for that matter. Hey, why don't you try Ubuntu while you're at it?
Download and install Oracle's VirtualBox here: https://www.virtualbox.org
I created a video of myself installing the Windows 10 Technical Preview on VirtualBox.