What is emergency management, how did it develop, and how is it different from civil defense?

What is emergency management, how did it develop, and how is it different from civil defense?
Please cite or reference the textbook:Introduction to Homeland Security: Policy, Organization, and AdministrationWillard M. Oliver, Nancy E. Marion, Joshua B. Hill, 2015Jones and BartlettISBN.13: 978-1-284-04583-3