YouTube today has updated its search facility to include new features called Filter and Explore, which has been added to a new interface within the advanced search options. The new features now allow you to select multiple filters from a category and combine the related searches.
The new features have been added as part of the an experiment called YouTube Topics that tried to help users find videos. An example of how this works is provided after the jump.
For example, you can find long-form HD videos that are available in the WebM format.
If you click on one of the queries from the “Explore” section, you’ll replace the original query. A more interesting option is to mouse over a query and click the “plus” icon: this way, you’ll refine your original search.
YouTube adds both filters and refinements to the query and separates them by comma. A search for [slow motion, hd, webm, high speed camera] includes two filters that are treated separately: “hd” and “webm”.
Source : Google OS