Are you using YouTube to share your own videos? Want to upload and share a video privately without setting it to private? Use the new "unlisted" feature on the popular video sharing site. With this feature, you can mark your videos as "unlisted", which means only people who have the direct link to the video will be able to watch it. This is a great privacy option, specially for those who wants to upload their group project videos to YouTube.
A video marked "unlisted" won’t appear in any of YouTube’s public pages, in search results, on your personal channel or on the browse page. It means it is a private video? Yes, it is. But unlike the old "private" videos your viewers don’t need a YouTube account to watch it. Also there is no limit to the number of people who can watch an "unlisted" video. So, it is a perfect option for sharing your personal family videos or group project videos.
How to use the "unlisted" video option? As usual, upload your video to YouTube. Then check the "Unlisted" option from the privacy section and click the "Save Changes" button. You will get a link to your video for sharing with your friends, family or colleagues. Your video will be filtered from public pages as well as search results and only those who have the URL to your video will be able to view it.
Source: http://youtube-global.blogspot.com/2010/05/more-choice-for-users-unlisted-videos.html