A List Apart: Articles: Design Patterns: Faceted Navigation
http://www.alistapart.com/articles/design-patterns-faceted-navigation/

Also called guided navigation and faceted search, the faceted navigation model leverages metadata fields and values to provide users with visible options for clarifying and refining queries. Faceted navigation is arguably the most significant search innovation of the past decade.[2] It features an integrated, incremental search and browse experience that lets users begin with a classic keyword search and then scan a list of results.