Until now, there were only two options when you upload a video to YouTube – either make it public or make it private. Making it public meant anybody can view the video without logging into YouTube and the videos will appear on search results and recommended videos. Private videos is the other extreme – only specific YouTube users can view them. Today YouTube rolled out a third option, Unlisted, whose privacy option lies in between the two.
When you mark a video as Unlisted, only people who have the link to the video can watch it. It won’t appear in any of YouTube’s public pages, in search results, on your personal channel or on the browse page. It’s a private video, except you don’t need a YouTube account to watch it and there is no limit to the number of people who can view it. It’s like a semi-private video.
So initially you will have to give out the link to the video to whoever you want to share it with, but if one of your contact share the link with their friends, these people can watch it too. If you feel your privacy was compromised you can restrict the video at any time by returning to your account and marking the video as Private.
[via YouTube Blog]