Network Access Control (NAC) evaluates the status of endpoint devices such as PCs and laptops before allowing a network connection. This allows for policy-based control of devices attaching to the network. Some typical NAC policies evaluate variables such as OS version and patch level, the presence of an approved anti-virus product, the activity of problematic services such as peer-to-peer file sharing, or the conformity of client device identity and settings with IT governance standards. 802.1X provides a simple form of NAC with respect to port level security, but many NAC approaches extend policy rules to the OS or application levels.
References:
http://technet.microsoft.com/en-us/network/bb545879
http://features.techworld.com/security/3228744/a-guide-to-network-access-control-endpoint-security/