is the site map provider for Microsoft SharePoint Server
Many named instances of this site map provider are listed in the web.config file, and are also available through static properties on this class that the master pages in SharePoint Server 2010 deliver. SharePoint Server 2010 uses these named instances to provide data to the horizontal and vertical navigation menus. The application shares named instances across all Web requests on the current application domain. The class stores no session information. If necessary, session information is stored on the HttpContext class.
A question tagged portalsitemapprovider regards development and the implementation of PortalSiteMapProvider.
Reference: PortalSiteMapProvider class