Facets provide aggregated data based on a search query. Typical usage examples are tag clouds and faceted navigation which allows users to restrict search results to certain categories such as product types, price ranges or author names.
Facets are requested and retrieved along with regular search requests and it is important to note that they are scoped for the search query by default.
Elastic Search, which EPiServer Find is built upon, features a number of facets ranging from simple to advanced such as Terms, Range, Statistical and Geo Distance. The .NET client API for facets currently supports Terms, Histogram, Filter, Range and Geographical distance.
Facets and filters
Normally facets are affected by filters, just like search results. However, it is possible to retrieve facets generated without applying the same filters as those applied to search results using the FilterHits method. Therefore it's good to read up on the Filter and FilterHits methods as there is a significant difference between them when working with facets.