YouTube introduces a prototype feature "YouTube Topics on Search" on TestTube, that simply try to identify topics on YouTube using sources: "frequently used uploader keywords, common search queries, playlist names, and even sources outside of YouTube such as Wikipedia articles" and associate videos with them.
"When you search -- say for "obama" -- we suggest other related topics tied to videos that you might want to explore, such as "michelle obama" or "john mccain." You can click to get videos on these topics or you can find videos that contain both topics by clicking on (+) next to topic. This's handy for refining. For instance, try searching for "turkey" and you'll see "thanksgiving" as a refinement option further down the page," explains Google.
Here's how it works:
Explore it on Test Drive.