How will radar systems be used in FAA's NextGen ATC plans?
Under the ADS-B rule of the FAA aircraft operating in Classes A, B, and C airspace, as well as certain other specified classes of airspace
within the U.S. National Airspace System are required to operate ADS-B Out avionics. 

This means that these aircraft can be tracked without the use of radar. These ADS-B tracks will be more accurate than radar tracks and have a higher update rate. However for reasons of integrity, continuity of service and security, radar will continue to provide coverage. Currently a lot of airspace is covered by multiple radar layers, the introduction of ADS-B allows for a reduction in redundant radar coverage.

Aircraft not equipped with ADS-B (operating outside of class A,B, or C airspace) will be tracked using radar. The radar tracks will be broadcast on TIS-B (Traffic Information Service - Broadcast) so that aircraft with ADS-B In receivers detect these aircraft as well.