In Lightroom 2 Adobe combined the Find and Metadata Browser panels from Lightroom 1, added some new functionality, moved them to center stage in Grid view and named it the Library Filter bar. The result is a tremendous boost in usefulness when you want to find and filter out all but a specific set of photos. This is a fantastic way of leveraging the database (i.e. catalog file) under its hood.
Read Finding Photos with the Library Filter Bar over at peachpit.com.