Taxonomy, a powerful core module, gives your sites use of the organizational keywords known in other systems as categories, tags, or metadata. It allows you to connect, relate and classify your website’s content. In Drupal, these terms are gathered within "vocabularies." The Taxonomy module allows you to create, manage and apply those vocabularies.
The Simple hierarchical select module or SHS for short implements a widget element that allows users to drill down through a hierarchical list of terms within a vocabulary. The module implements a custom formatter, widget and Views filter. You can create a views listing page that allows users to filter content using SHS. The module only works with vocabulary terms so you'll need to have Taxonomy enabled to use this module.