A List Apart recently posted an excerpt from Chapter 4 of Search Patterns by Peter Morville and Jeffery Callender that presents a great description of facetted navigation (FN), a type of search interface.
FN is contrasted with just text searching (e.g., Google), taxonomies (e.g., Windows Explorer or Mac Finder), and tag-based interfaces (e.g., Flickr). See illustrative figure below if you aren’t familiar with these types of interfaces.
My most recent encounter was at the online shoe store Zappos. When looking for some shoes, the user is presented with a very dynamic FN: