YouTube is currently experimenting with a new search results page that lets you refine your results using popular tags. Say, in this example, if when search [race car], YouTube displays a list of related tags that relates to your search query (pic 1).
YouTube also shows a list of tags for each search result. When you click any one of the tags, you'll restrict the list of search results to videos that have a certain tag (pic 2). YouTube not only use "user-defined tags," but some of the tags are "video properties, tags used in comments, and also the usernames".(pic 3)