If you want to link to a youtube video and you want people to see the high definition (HD) version of the video, just copy the URL as you would normally. Then, append &hd=1 to the end of the URL. When someone clicks on this link, they'll get taken to the high definition version of the video instead of the standard quality version.

If you're going to do this though, make sure an HD version of the video exists. If not, this link will not work and they will not get taken to the expected video on youtube.