Today Youtube has officially announced their latest feature in new version of video player, "Thumbnail Storyboard Feature". Using this function you can see the snapshot of video at current time when cursor is pointed in the video time graph (I Had Seen This Feature When Surfing Some Videos In Last Month). This feature is not currently available to all users.
Actually Youtube makes so much number of thumbnails for one video and when we point cursor on the time, Youtube will check out and display the corresponding image with 10x10 Pixel size. So you can easily know what the video consists.