YouTube experiments with a new search results page that lets you refine results by selecting related tags. For example, if you search for [race car], YouTube shows a list of popular tags related to your query: #autos & vehicles, #indycar, #cars, #wreck, #car crashes etc. YouTube also shows the list of tags for each search result.
If you click on one of the tags, you’ll restrict the list of search results to videos that have a certain tag.
YouTube doesn’t use only user-defined tags, some of the tags are video properties (#hd, #fresh), tags used in comments (#omg, #lol, #wtf, #ftw, #fail, #cute), usernames.
