The Facet pane shows the relevant criteria by which to navigate and refine the search results. We refer to these criteria as facets, and the concept of using facets to navigate and refine search results as a "faceted search".
In introducing the concept of a faceted search, it may be helpful to draw the distinction between it and a more traditional filter-based search.
Filter-based search
Filters work by analyzing the search results and excluding resources (for example, assets, users and communities) that don't meet certain criteria.
In a filter-based search, the collection of filters that is available to you is constant. The relevance of the filters to the search results is not considered. As such, some of the filters available to you could either have no effect on the results, or reduce the results to a null set.
Faceted search
Facets are dynamically generated according to the resources that make up the search results. They allow you to narrow results by several different dimensions simultaneously.
In a faceted search, only facets that are relevant to the search results appear in the Facet pane on the Search page. As you select facets, to navigate and refine the search results, the Facet pane is automatically updated, again showing only facets that are relevant to the refined set of results.
As shown in the following image, the facets in the Facet pane at any given moment include the number of resources that match both the search text and the value of the available facets. This gives you a view of the composition of the search results, and helps you avoid searches that yield no results.