This post covers the various components of NAP [Network Access Protection (NAP)].  The diagram below from the NAP Architecture Whitepaper (the link to the Whitepaper is at the end of this post) shows the various components of a NAP-enabled infrastructure:

The components are as follows:

• NAP Clients: Client systems that support the NAP platform – Windows Server 2008, Windows Vista and Windows XP SP3
• NAP Enforcement points: NAP enforcement points evaluate the health of NAP clients, determine whether network access or communication is allowed and what remediation actions (if any) a non-compliant NAP system must take.  Examples of NAP enforcement points include VPN Servers (Windows Server 2008 with RRAS), DHCP Servers (Windows Server 2008) and Network Access devices such as Ethernet or  Wireless Access Points that support IEEE 802.1x authentication […]

Full Article

Additional Resources:

• Network Access Protection Platform Architecture Whitepaper
• Configuring the TS Gateway NAP Scenario
• MSDN: Network Access Protection (contains links to API References)