Hi all How does having a server in a DMZ provide more security than placing it behind a firewall? please explain

http://en.wikipedia.org/wiki/DMZ_(computing)

Hi, A demilitarized zone (DMZ) is a network that permits the egression of the Internet into a private network. It is typically a separate isolated private network which connect the internal network and the public network. DMZ helps to establish the security boundary between the public network and the internal. If the public access servers get hacked in DMZ, internal resources are still under protection while those resource are exposed to the intruder if you simply put the public server behind firewall.

thanks Miles Li