What is faceted search?

Faceted search is well-explained here and Lucene faceted through an example.

EXAMPLE:

Faceted (or navigational) search uses a hierarchy structure (taxonomy) to enable users to browse information by choosing from a pre-determined set of categories. This allows a user to type in their simple query, then refine their search options by navigating/drilling down. In reality, it's an advanced search going on in the background, but instead of the user having to think of the additional search categories, it's been made easier for them by the visible folder structure. Examples of this are eCommerce sites like Amazon and eBay.