Following recent reports Windows 10 was attempting to self-install on some systems, Microsoft assured the public this was a mistake and would be rectified in short order. That's true (the upgrade will be flagged as an "Optional Update" now or soon), but come early 2016, Windows 10 will officially adopt this behavior when it becomes a recommended update.
To be clear, Windows 10 will not automatically install itself, it will simply begin the process, at which point you can stop it or continue. As well, you can change this behavior and whether or not you see the notification upgrades via settings. Lastly, you'll have the option to roll back to your previous version of Windows within 31 days of installing Windows 10.