It's the new ground breaking security feature in Server 2008 (Even though other company's have made this product already). It helps protect networks from internal threats, by making a user validate before their given pysical network access, and enforcing policys such as making sure the client has the latest anti-virus definitions.
You can read about it here
http://en.wikipedia.org/wiki/Network_Access_Control
And here's Microsoft implementation
http://en.wikipedia.org/wiki/Network_Access_Protection