Mike Padilla offers up a description of facets and their use on web sites for browsing and filtering. He defines the types of interactions or behaviors that are typically found on sites with faceted browsing, describes what happens in terms of the system when you apply filters, and shows some examples of faceted browsing in use on sites including my favorite, Nike.com.
http://www.digital-web.com/articles/user_interface_implementations_of_faceted_browsing/