YouTube's Autoplay feature automatically plays suggested videos automatically after the video you requested ends

You can choose to toggle this functionality off if you'd like. YouTube will remember your preference. As a useful side effect, YouTube will probably also remember you which is great for tracking and analytics. If you're not already logging into your Google account, you're losing out on great personalization and customization.

Further reading: http://techcrunch.com/2014/12/11/youtube-is-testing-an-autoplay-feature-that-streams-suggested-videos-automatically/