On Nov'10th at around 6.00 pm pacific time, YouTube will be making YouTube Topic Search available through its TestTube "labs" area, that allows people to easily explore videos by topic.
The ultimate goal is to recommend videos to users while also giving them some context explaining why the video is being recommended. For e.g, you can search for videos and then narrow them down into areas such as "LOL" to find those that're especially funny. But if you're not interested in cats, you can simply "X" out that Topic and further hone in on the kind of content you'd like to see.
The screenshot below, give you a clear picture of the new feature:
When you select one of the Explore topics, you get a new collection of videos that match what you originally searched on and narrowed to (IE, above, clicking on "miley cyrus" would give you videos matching her and "lady gaga," and the videos themselves will be tagged -- Bad Romance has a tag for "Workout song", for e.g. Clicking that "Workout" tag will take you to a list of other videos that YouTube believes shares this attribute, and each of these videos will have its own Topic tags to let you get even more specific.