In order to allow friends to share their favorite parts of videos, YouTube announced the ability to "deep link" to YouTube videos. This means you can now not only link to a YouTube video itself, but you can also link directly to a specific time within each video.
Google says that in order to create a deep link, you have to add "#t=1m15s" to the end of a YouTube video URL. So your whole link would be something like http://www.youtube.com/videoID#t1m15s. This sets the link to the time 1:15 in the video. Of course, you can replace the numbers before the 'm' and the 's' with anything you like, setting up the link to any part of the video you wish.
Google will also automatically create links if a viewer mentions a specific time in the comments section. If someone writes, "LOL, 1:15 is hysterical", this will become a hyperlink. Other viewers will then be able to click on that link and jump to that moment in the video.
Thanks, Google. Keep the good stuff coming.