All the types, topics, and other terms classifying content on a Drutopia site can be used to narrow down what you are shown. Faceted search powers this functionality.
Faceted search enhances full text search (also sometimes called keyword search), or it can be used as a replacement for it.
A well-implemented faceted search, such as Drutopia does, means that as people filter the search results they do not reach dead ends.
You can see faceted search at work here in Drutopia.org's resources section.