Microsoft has released a preview of the upcoming Windows version, Windows 10 (codenamed Windows Threshold), for Windows enthusiasts to download and provide feedback.
In the preview, you can't turn off Windows Update which means Microsoft can remotely push updates to your Windows installation. Newer Windows 10 builds are going to be released in the future so this build has an expiration date of April 15, 2015 after which you won't be able to use it. You can now also join the Windows Insider Program where you can access more benefits and help shape Windows 10.
Before using Windows 10 as your main installation, be sure to check out the Privacy Statement. This time Microsoft is collecting more data than you would want it to, like:
"[we collect data when you] enter text, we may collect typed characters and use them for purposes such as improving autocomplete and spellcheck features."