Provides faceted indexing and search capabilities. Checkout this and this blog posts for some overview on the facets module as well as source code examples here.
Packages 
Package Description
org.apache.lucene.facet
Faceted search.
org.apache.lucene.facet.facetset
Provides FacetSets faceting capabilities which allows users to facet on high dimensional field values.
org.apache.lucene.facet.range
Provides range faceting capabilities.
org.apache.lucene.facet.sortedset
Provides faceting capabilities over facets that were indexed with SortedSetDocValuesFacetField.
org.apache.lucene.facet.taxonomy
Taxonomy of Categories.
org.apache.lucene.facet.taxonomy.directory
Taxonomy index implementation using on top of a Directory.
org.apache.lucene.facet.taxonomy.writercache
Improves indexing time by caching a map of CategoryPath to their Ordinal.