The website for the Elizabeth Hartwell Mason Neck National Wildlife Refuge is http://www.fws.gov/refuge/Mason_Neck/ while the URL for the Mason Neck State Park is http://www.fws.gov/refuge/Mason_Neck/.
The first difference is that the refuge is primarily meant to protect the environment while the park is meant to be a place for people to have fun and do activities. The second difference is why they were created. The refuge was first created federally to protect the Bald Eagle, while the park was created by the state of Virginia to be a State Park. Another big difference is the governing body of each place. The Virginia Department of Conservation and Recreation governs the park while The US Fish and Wildlife Service governs the refuge.