NPS is a role service in Windows Server 2008 which can function as:
NPSEE enables the use of a heterogeneous set of wireless, switch, remote access, or VPN equipment. One can use NPS with the Routing and Remote Access service, which is available in Microsoft Windows 2000, Windows Server 2003, Standard Edition; Windows Server 2003, Enterprise Edition; and Windows Server 2003, Datacenter Edition.2
When a server running NPS is a member of an Active Directory Domain Services (AD DS) domain, NPS uses the directory service as its user account database and is part of a single sign-on solution. The same set of credentials is used for network access control (authenticating and authorizing access to a network) and to log on to an AD DS domain.3
"Network Policy and Access Services". 2 July 2012. https://technet.microsoft.com/library/cc754521.aspx ↩
"Network Policy Server Overview". 22 October 2013. https://msdn.microsoft.com/en-us/library/cc771347.aspx ↩